make VARY_LOW and LOW non fatal

[jacekjaros]

Is your feature request related to a problem? Please describe.
I want to use salt-lint to check salt formulas syntax before deployment to salt servers. My intention here is to find only fatal errors and not to force 100% syntax to be fixed. Issue is here that salt-lint is return the same exit code regardless if problem is LOW or CRITICAL.

Describe the solution you'd like
User should have ability to make selected severity non-fatal.

Describe alternatives you've considered
There should be cli parameter like --non-fatal taking list on severitys which not cause non-zero exit code.