Add --<lang>_opt flag for all built-in generators

[Yannic]

Plugins (and some built-in generators) allow passing parameters
with --<lang>_opt one-by-one instead of passing them as
--<lang>_out=<params>:<out_base>. This PR changes protoc to
allow using --<lang>_opt for all (built-in) generators.

[Yannic]

Thanks! The failures look unrelated to this change.

macOS Python Release:

+ sudo installer -pkg downloads/python-3.5.4-macosx10.9.pkg -target /
installer: Error - the package path specified was invalid: 'downloads/python-3.5.4-macosx10.9.pkg'.


[ID: 4240753] Build finished after 622 secs, exit value: 1

macOS Ruby 2.3

Ruby was built without documentation, to build it run: rvm docs generate-ri
Using /Users/kbuilder/.rvm/gems/ruby-2.3.8
/Users/kbuilder/.rvm/rubies/ruby-2.3.8/bin/ruby
ERROR:  Loading command: install (LoadError)
	cannot load such file -- openssl
ERROR:  While executing gem ... (NoMethodError)
    undefined method `invoke_with_build_args' for nil:NilClass


[ID: 2752210] Build finished after 705 secs, exit value: 1

macOS Ruby Release timed out, not sure why but I'm sure not because of this change.

[acozzette]

@Yannic Thanks for the pull request.