--fail-level command line option does not accept severity names, only codes

```
➜  git:(add-erb-lint) ✗ erb_lint --lint-all --fail-level=fatal
fatal: not a valid failure level (info, refactor, convention, warning, error, fatal)

➜  git:(add-erb-lint) ✗ erb_lint --lint-all --fail-level=F
Linting 887 files with 1 linters...
...
73 error(s) were ignored in ERB files
```
