Adding show-toolchain for command line prompts.

[aetherknight]

This implements a command to satisfy #147 .

It adds multirust show-toolchain, which prints just the name of the current toolchain. Eg, "nightly", or "stable".

[aetherknight]

Should it print something if it can't find a default, or should it print nothing?

[aetherknight]

For comparison, rbenv local prints nothing to STDOUT, but it does print an error to STDERR if it cannot determine the local version of ruby that should apply.

[brson]

Let's print nothing then to match rbenv.

[brson]

I hate that show-override, show-default and show-toolchain are all slightly different. rbenv has just one version subcommand, and that's probably what I'm going to implement in multirust-rs.

I guess this is consistent with the existing multirust interface though.

For your purposes could you use sed to parse this out of the show-override command? Edit: nevermind I see you already addressed that in the op.

[brson]

I went ahead and merged as-is. Thanks @aetherknight!