Add severity to standard output. Fixes #93

[mew1033]

I added this as an argument that defaults to off so that the output stays consistent for people. If you want it, the flag is --add-severity.

[roaldnefs]

Thanks @mew1033 for your pull request!

The salt-lint package is heavily based upon ansible-lint and I therefore would prefer a approach similar to ansible-lint.

The flag itself is defined as --parseable-severity in ansible-lint (https://github.com/ansible/ansible-lint/blob/master/lib/ansiblelint/__main__.py#L68), because the used formatter in your pull request isn't the parseable formatter I would prefer the flag --severity over --add-severity.

In addition to changing the flag, I would like to see a separate formatter e.g. SeverityFormatter as this doesn't required the add_severity parameter to be passed to the default Formatter in saltlint/formatters/__init__.py.

An example of the ParseableSeverityFormatter in ansible-lint can be found here: https://github.com/ansible/ansible-lint/blob/ea4cd39bbcc508a96c17d9e97f12278961023487/lib/ansiblelint/formatters/__init__.py#L59. The same file also includes a ParseableFormatter.

[mew1033]

@roaldnefs I've made changes based on your comments. Thanks!