Apply `defcustom` fixes

[wasamasa]

I find it weird that inf-ruby allows one to select the Ruby implementation interactively, yet it hasn't a defcustom form to choose the default Ruby implementation to fall back. Additionally to that I've added extra code to make customize construct a proper interface from the two forms used.

[wasamasa]

Perhaps add inf-ruby-implementations, too?

[dgutov]

Perhaps the possible values of inf-ruby-default-implementation should depend on the available keys in inf-ruby-implementations?

Perhaps add inf-ruby-implementations, too?

Maybe. It seems a bit more complicated, though.

[wasamasa]

This implements the first one, as for the second one, I'm less sure how one would fit that into customize.

[dgutov]

as for the second one, I'm less sure how one would fit that into customize

Check out http://www.gnu.org/software/emacs/manual/html_node/elisp/Composite-Types.html#Composite-Types

You should be able to combine :type, repeat, cons and string to make it work.

[wasamasa]

This should address the problem.

[dgutov]

Thanks!