Differentiate between errors and lints

Previously, `haml-lint` would report syntax errors just like any other
kind of lint. This becomes problematic when integrating with tools like
Syntastic (https://github.com/scrooloose/syntastic), as they treat each
type of error differently (errors are something you _must_ fix, whereas
warnings are something you _should_ fix).

Add a severity level to the `Lint` class and include that severity in
the output.

Change-Id: I1288ba40cd2e760617145a61b63ecbc3b675d056
Reviewed-on: https://gerrit.causes.com/30954
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>