Port over counter implementation and introduce linter against title attribute

[khiga8]

Closes https://github.com/github/accessibility/issues/646, #18

Context

title attribute is frequently used though it is inaccessible to many groups of users and hardly useful. This PR introduces a linter rule against it. This rule is implemented as a counter unlike the other rules, though we may consider converting other rules to counters too. This means that if a user wants to disable this rule on a file, they need to disable it with the exact offense count like:

<%# erblint:counter GitHub::Accessibility::FakeCounter 1 %>

instead of

<%# erblint:disable GitHub::Accessibility::FakeLinter %>

The counter implementation is a workaround ported over from dotcom. erb-lint does not natively support granular rule disables so in dotcom we've resorted to a file-level offense implementation and a counter implementation (both of which this library uses too). Having this workaround isn't great.

I've opened a proposal PR to shopify/erb-lint (Shopify/erb_lint#249) with the hopes that we can move away from these workarounds and have disable rule at offense-level like rubocop and eslint allows. Until we have that support, we'll need to work with our workarounds with file-level disable and counters

[lindseywild]

I don't feel comfortable approving because I don't fully understand the ERB linters just yet, but just a few suggestions on the docs -- thanks for doing this 😄

[bolonio]

From not being an erb expert, it looks good to me, if the tests have been passed, feel free to release and keep an eye on your PR for Shopify.