Add no-keyword-prefix rule

[MrHen]

Fixes #251.

This is an initial PR to judge interest. As I mentioned in the issue, I was planning on writing this rule anyway and figured I would submit it to unicorn instead of building a one-off package.

I don't expect this to be a popular rule -- it's merely solving a nitpick I've had on my existing team's code style.

There is some remaining work to do but I didn't want to polish it without knowing if the PR would be accepted.

• Document configuration options
• Update rule listing in readme
• Update recommended config
• Update recommended config in readme
• Resolve TODOs (mostly missing tests)
• Split up the Identifier function into smaller, easier to digest chunks

[sindresorhus]

While I won't use this rule personally, I'm happy to host it here if you can commit to maintain it. I think it should be "off" by default in the recommended config though.

[MrHen]

@sindresorhus This PR is now ready for review.

Relatedly, I'm not sure what your preferred process is for reviewing / merging PRs on your repositories. I've been assuming you want to review every PR and will decide when to merge them. Is that correct?

[sindresorhus]

Relatedly, I'm not sure what your preferred process is for reviewing / merging PRs on your repositories. I've been assuming you want to review every PR and will decide when to merge them. Is that correct?

It doesn't have to be me, but it's always useful to get a second set of eyes on the code.

[sindresorhus]

Looks good to me. I'm gonna leave this open for a day to give other people a chance to review if they want.