only and --rules/--invalid/--valid

[brettz9]

• Enhancement: Allow only on test cases or CLI comma-separated --rules and comma-separated --valid=idxOrNegIdx/--invalid=idxOrNegIdx

Fixes #13.

1. I've implemented such that only only works if --rules is present. I can implement it to work without it, but I thought it was helpful to avoid the extra overhead it necessitates by default.

2. Note that without a means in ava to dynamically ignore test files, one will still get notices about test files not having tests (for those files where all test cases are filtered out).

3. Note also that I am checking for 3 config sources. One was added to allow for easier unit testing (within the constructor), and one (including the minimist dependency) was added so as to support regular CLI usage or the double-double-hyphen syntax npm test -- --rule described at https://github.com/avajs/ava/blob/master/docs/recipes/passing-arguments-to-your-test-files.md . However--and ava could be modified to support this too--you don't actually need the extra double-hyphen if you are running from package.json scripts and if the code checks for process.env.npm_config_*, e.g., process.env.npm_config_rules, as I've done in this PR also.

I've also added this which I can remove or alter as needed:

• npm: Bump peerDeps (eslint) and devDeps (xo)

[brettz9]

Any chance you can take a look at this?

[brettz9]

@sindresorhus : Are you involved on this project? If so, as it ties in to sindresorhus/eslint-plugin-unicorn#501 , an issue you favored, I was hoping you might be able to take a look?