Update linters to load custom linters

[jen-taylor-shopify]

Closes #164

This PR builds on a suggestion by @etiennebarrie in this PR.

The goal is to update the linter_registry so that it also loads the custom linters when the options are parsed. This prevents not a valid linter name errors when attempting to run erb-lint with a specific custom linter:

$ bundle exec erblint --enable-linters custom_linter --lint-all
custom_linter: not a valid linter name (rubocop, rubocop_text, parser_errors, space_around_erb_tag, space_in_html_tag, extra_newline, right_trim, erb_safety, trailing_whitespace, closing_erb_tag_indent, space_indentation, hard_coded_string, no_javascript_tag_helper, allowed_script_type, self_closing_tag, final_newline, deprecated_classes)

A lot of the "why" is detailed in #164, but this seemed like the shortest lift.

[etiennebarrie]

The test needs to be fixed, but it does help running a custom linter with --enable-linters.

[rafaelfranca]