Subshells

Support for subshells, e.g. `:echo "In $(!pwd)"`

The current parser rules don't support nesting, which might be a blocker (see #198), but I wanted to open a PR with this before it got too huge.

(Specs pass locally, Travis is suffering from the usual Readline segfault shenanigans)
Do you need to call `super` here? It looks like the default implementation does some sanity checks: https://github.com/ruby/ruby/blob/61ad543a33fe10d21d1dc85efcbf3e9d96cc93b7/object.c#L429