Fix hex color linter to work in Sass script

The hex linter worked in situations where the hex color appeared as the
value of a property declaration, e.g.

    p {
      color: #112233; // Lint reported
    }

However, it would not work in situations where the hex color appeared as
arguments to a function or mixin, e.g.

   p {
     @include some-mixin(#112233); // Should report lint, but doesn't
   }

Even with the new support for traversing the Sass script parse tree, we
still can't fix this because the parser normalizes colors it encounters,
so we don't have access to the original characters that were written.

The only solution is to then do a line-by-line regex check for hex
colors (i.e. ignoring the parse tree) and report lints that way.

Fixes #7

Change-Id: Ib0e7ddddc221bc00a3b3677f3bc78d8e89d50f45
Reviewed-on: https://gerrit.causes.com/23185
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>