Ignore case of keywords for valid_docs

[aciidgh]

Fixes #298

[jpsim]

Thanks for the contribution! Could you please add a changelog entry following the guidelines in our CONTRIBUTING.md document? Then we'll be able to merge this.

[aciidgh]

@jpsim Done.

[jpsim]

Thanks so much! My testing confirms this behaves as expected, with only a minimal performance impact.

Linting Carthage 0.11 spends 2.673s in valid_docs on SwiftLint master, vs 2.696s in this PR.

Although more generally valid_docs is by far the slowest rule in SwiftLint (followed by colon at 1.5s and function_parameter_count at 0.7s). The rest mostly are all under 0.2s.