* Keep include prefix when converting from proto.

This change preserves the include prefix when generating flatbuffers
from proto (with FBS_GEN_INCLUDES) defined.

* Improve handling of imports in proto conversion.

Previously, there was no runtime flag to make proto->fbs conversion keep
the import structure of a collection of files. This change makes proto
conversion respect the --no-gen-includes flag and skip the output of
"generated" symbols.