Runtime protocol: send list of supported component languages

[bergie]

Now noflo-ui uses a per-runtime-type hard-coded list of languages. However, in some cases this may depend on the installation. For example:

• NoFlo may or may not support CoffeeScript depending on whether the CoffeeScript compiler is present
• MsgFlo supports lots of languages, but only if their "runners" are installed

So, runtime:runtime message could return an array of supported languages.

Needed for noflo/noflo-ui#206