* 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