Regular expressions

[kddnewton]

We need to finish parsing regular expressions.

• At the moment we handle regex but only if there's no interpolation. We need to also handle interpolation.

• We also need to handle regular expressions with named captures creating local variables. We have a parser here that can handle that, but we haven't hooked it up yet. That looks like:

/aaa (?<foo>bbb) ccc/ =~ "..."
foo

• We also need to handle %r regular expressions. At the moment we're only handling regular expressions that start with /.

For documentation, check https://github.com/ruby/ruby/blob/master/doc/syntax/literals.rdoc#label-Regexp+Literals and https://github.com/ruby/ruby/blob/master/doc/syntax/literals.rdoc#r-regexp-literals-.