Add commands to browse description in Forth standards

[ellerh]

The keys C-c C-d 1 resp. C-c C-d 2 invoke the command to lookup a word in the '94 resp. 2012 version of the standard.

• forth-spec.el: New file.
• forth-mode.el: Load it and bind keys.
• test/tests.el: Add tests.

[ellerh]

I'm not merging this myself in case you don't like it.

[larsbrinkhoff]

I do like it! The key bindings look a little bit quirky, though, with the 1 or 2 to select a standard.

This fixes #10, right?

[larsbrinkhoff]

I'd like to see a successfuld build status once dockerproject.org unwedges.

[ellerh]

This fixes #10, right?

Yes, right.