bin/pegjs: Output just the parser source if --export-var is empty
bin/pegjs: Output just the parser source if --export-var is empty

This will make embedding generated parsers into other files easier.

Based on a patch by Glen Huang:

  https://github.com/dmajda/pegjs/pull/143

wwall/pegjs-fn,pegjs/pegjs,crguezl/pegjs,Mingun/pegjs,mcanthony/pegjs,jlturner/pegjs,Sciumo/pegjs,wwall/pegjs-fn,pegjs/pegjs,mcanthony/pegjs,crguezl/pegjs,luisvt/pegjs,GerHobbelt/pegjs,jlturner/pegjs,luisvt/pegjs,GerHobbelt/pegjs,Mingun/pegjs,Sciumo/pegjs