Add support for traversing Sass::Script::Nodes

The implementation of the Sass parser involves two different kinds of
parse trees: ones composed of `Sass::Tree::Node`s and ones composed of
`Sass::Script::Node`s. The `Sass::Tree::Node` hierarchy encodes the
structure of SCSS documents, whereas the `Sass::Script::Node` captures
the parse tree of various Sass script throughout the document (e.g.
variables, string interpolation, argument lists, etc).

Unfortunately, this architecture means there is no built-in support to
have a visitor visit both types of nodes--only `Sass::Tree::Node`s
support visitors.

In order to give us greater flexibility in the kind of lints we can
write, we need to monkey-patch the Sass parse tree hierarchy to support
traversal over both tree and script nodes.

The net result is that lints can now define methods like
`visit_script_variable` or `visit_script_list` to easily find lints
for those kinds of nodes.

Change-Id: I60797e3748ddb5926674e750cc184da109a23cc0
Reviewed-on: https://gerrit.causes.com/23141
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>