Differentiate syntax errors from lints

In order for `scss-lint` to be usable when integrated with a plugin like
Syntastic (https://github.com/scrooloose/syntastic), it needs to
differentiate between syntax errors (i.e. problems that _must_ be fixed)
and lint warnings (i.e. problems that _should_ be fixed).

Add a `severity` level to the `Lint` class which defaults to `warning`,
and set the severity level of `Lint`s created by syntax errors to
`error`.

Change-Id: I52608a3156aec7d6c5caaa26fe1103ead7af7852
Reviewed-on: https://gerrit.causes.com/29749
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>