parse_source: allow direct targetting of a node

[marcandre]

Many tests in rubocop-ast need to generate an AST from Ruby code and then select an inner node.

It is very difficult to tell when reading the test which node it is:

# before:
let(:send_node) { parse_source(source).ast.children[2].children[0] }
let(:source) do
  ['class Foo',
   '  bar :baz',
   '  bar :qux',
   'end'].join("\n")
end

With this PR, we can target a particular node by wrapping it between '>>' and '<<' and parse_source can then target it by matching the source:

# after:
let(:source) do
  ['class Foo',
   '>>bar :baz<<',
   '  bar :qux',
   'end'].join("\n")
end