Make gflags and boost::program_options play nice with each other

Make gflags and boost::program_options play nice with each other

Summary: GFlags is useful for global program options. boost::program_options makes it
easier to write command-line utilities. They don't, unfortunately, play
very nicely with each other.

Add a addGFlags() function to convert all GFlags to boost::program_options
options; you can then use boost::program_options::parse_command_line() to
parse all arguments, GFlags or not.

Also add a helper function to make parsing nested command lines easier.

Reviewed By: @fugalh

Differential Revision: D2215285