* Add --force-defaults option to flatc

To emit default values for fields which are not present or which are set
to the default value.

* flatc option --force-defaults should have a default value (false) and take action on the builder_ within the Parser constructor

* Add help text from flatc --force-defaults to Compiler.md doc

* Clarified docs for flatc --force-defaults, and imply that this behaviour is not normally needed.

* Updated docs and flatc help text for --force-defaults option