Add NodeTransformer

Add a class to manage any special cases that come up with transforming
`Haml::Parser::ParseNode`s to `HamlLint::Tree::Node`s. While there are
none right now, this work was done anyway because we thought there might
be one.

For now, leave this in place.

Change-Id: I257388cd49245324487e5515a64e543cb520f51d
Reviewed-on: http://gerrit.causes.com/42564
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>