Allow some way of marking lints as TODO

I'd like there to be some way to gradually introduce clippy to a code base without having to fix all the existing problems but also minimizing or preventing new violations.

My preferred workflow when introducing new lints to an existing code base is:

1. Install the tool.
2. Have the tool generate a "TODO" file that marks all existing lint violations as ignored.

• Ideally this file is split into two dimensions: file and lint type.

At this point, I commit and push this progress. This prevents any files from getting worse. Then, the team cycles on the following steps:

1. Pick a single lint that is being ignored.
2. Fix all of or a subset of the files for that one lint, and push it.

What's interesting to note is that this entire process usually repeats when a new version of the linting tool is released, as this often introduces new lints that we have been unknowingly violating.

This is strongly influenced by RuboCop.

/cc #1295, killercup/rustfix#36

Start by reviewing the workflow described in this issue, along with the related discussions in #1295 and killercup/rustfix#36 and RuboCop's approach. Define how a generated TODO file would record existing violations by file and lint type, while preventing new violations, and verify the design against new lint versions.