Generalize plugin support in Bazel Skylark rule

It helps users to support their own plugins in a manner consistent to
cc_proto_library and py_proto_libary in their skylark rules

protobuf.bzl

@@ -62,9 +62,19 @@ def _proto_gen_impl(ctx):
   if ctx.attr.gen_py:
     args += ["--python_out=" + ctx.var["GENDIR"] + "/" + gen_dir]
 
-  if ctx.executable.grpc_cpp_plugin:
-    args += ["--plugin=protoc-gen-grpc=" + ctx.executable.grpc_cpp_plugin.path]
-    args += ["--grpc_out=" + ctx.var["GENDIR"] + "/" + gen_dir]
+  if ctx.executable.plugin:
+    plugin = ctx.executable.plugin
+    lang = ctx.attr.plugin_language
+    if not lang and plugin.basename.startswith('protoc-gen-'):
+      lang = plugin.basename[len('protoc-gen-'):]
+    if not lang:
+      fail("cannot infer the target language of plugin", "plugin_language")
+
+    outdir = ctx.var["GENDIR"] + "/" + gen_dir
+    if ctx.attr.plugin_options:
+      outdir = ",".join(ctx.attr.plugin_options) + ":" + outdir
+    args += ["--plugin=protoc-gen-%s=%s" % (lang, plugin.path)]
+    args += ["--%s_out=%s" % (lang, outdir)]
 
   if args:
     ctx.action(
@@ -72,6 +82,7 @@ def _proto_gen_impl(ctx):
         outputs=ctx.outputs.outs,
         arguments=args + import_flags + [s.path for s in srcs],
         executable=ctx.executable.protoc,
+        mnemonic="ProtoCompile",
     )
 
   return struct(
@@ -82,7 +93,7 @@ def _proto_gen_impl(ctx):
       ),
   )
 
-_proto_gen = rule(
+proto_gen = rule(
     attrs = {
         "srcs": attr.label_list(allow_files = True),
         "deps": attr.label_list(providers = ["proto"]),
@@ -93,11 +104,13 @@ _proto_gen = rule(
             single_file = True,
             mandatory = True,
         ),
-        "grpc_cpp_plugin": attr.label(
+        "plugin": attr.label(
             cfg = HOST_CFG,
+            allow_files = True,
             executable = True,
-            single_file = True,
         ),
+        "plugin_language": attr.string(),
+        "plugin_options": attr.string_list(),
         "gen_cc": attr.bool(),
         "gen_py": attr.bool(),
         "outs": attr.output_list(),
@@ -105,6 +118,26 @@ _proto_gen = rule(
     output_to_genfiles = True,
     implementation = _proto_gen_impl,
 )
+"""Generates codes from Protocol Buffers definitions.
+
+This rule helps you to implement Skylark macros specific to the target
+language. You should prefer more specific `cc_proto_library `,
+`py_proto_library` and others unless you are adding such wrapper macros.
+
+Args:
+  srcs: Protocol Buffers definition files (.proto) to run the protocol compiler
+    against.
+  deps: a list of dependency labels; must be other proto libraries.
+  includes: a list of include paths to .proto files.
+  protoc: the label of the protocol compiler to generate the sources.
+  plugin: the label of the protocol compiler plugin to be passed to the protocol
+    compiler.
+  plugin_language: the language of the generated sources
+  plugin_options: a list of options to be passed to the plugin
+  gen_cc: generates C++ sources in addition to the ones from the plugin. 
+  gen_py: generates Python sources in addition to the ones from the plugin.
+  outs: a list of labels of the expected outputs from the protocol compiler.
+"""
 
 def cc_proto_library(
         name,
@@ -150,7 +183,7 @@ def cc_proto_library(
   if internal_bootstrap_hack:
     # For pre-checked-in generated files, we add the internal_bootstrap_hack
     # which will skip the codegen action.
-    _proto_gen(
+    proto_gen(
         name=name + "_genproto",
         srcs=srcs,
         deps=[s + "_genproto" for s in deps],
@@ -170,13 +203,14 @@ def cc_proto_library(
 
   outs = _CcOuts(srcs, use_grpc_plugin)
 
-  _proto_gen(
+  proto_gen(
       name=name + "_genproto",
       srcs=srcs,
       deps=[s + "_genproto" for s in deps],
       includes=includes,
       protoc=protoc,
-      grpc_cpp_plugin=grpc_cpp_plugin,
+      plugin=grpc_cpp_plugin,
+      plugin_language="grpc",
       gen_cc=1,
       outs=outs,
       visibility=["//visibility:public"],
@@ -286,7 +320,7 @@ def py_proto_library(
   if include != None:
     includes = [include]
 
-  _proto_gen(
+  proto_gen(
       name=name + "_genproto",
       srcs=srcs,
       deps=[s + "_genproto" for s in deps],