Add severity level support like rubocop

[domingo2000]

Currently this gem does not allow a severity level of the linters in the .erb-lint.yml configuration. This is useful for incremental inclusion of linters in a big repository, because when a new linter is inrtoduced is difficult to correct all the offenses at one time.

It should be nice to introduce severity levels as rubocop described here.

As I tested the linter allows the rubocop severity levels, but does not have implemented severity levels in the html.erb specific linters.

If you have an idea of implementation you can also tell me and i be glad to help.