Support for non-string option value assignment

Support for non-string option value assignment

Make it possible to assign non-string values to options through
OptionsAssigner.  For now, the only thing that actually works is
assigning std::string values, and assigning the exact type stored for
the option.  Also, the list of supported conversions is initialized
separately for every instance of each option, but in practice that might
not matter that much.

This adds the infrastructure that allows assigning option values from
something like a JSON structure that already does data type conversions
during parsing.  The same infrastructure can also support #653.

Change-Id: I8f81d57da1b6eeac06a6ac9a8ab68bfa8ed0c148