Add an easy way to run clippy with high FPR / trivial lints disabled

Hi!

It often happens that users submit a "make clippy happier" PR to one of my projects. These PRs often contain quite a bunch of changes, of which a couple are undeniably great, but majority just shuffle code around. An example of good change would be a removal of (now useless) .into() or .clone(). A typical example of not really good change is .unwrap_or(xs.len()) -> .unwrap_or_else(|| xs.len()).

I've written at some length about this here: https://github.com/rust-analyzer/rowan/pull/57#discussion_r415676159.

I would really love the ability to run cargo clippy --conservative, and only get lints about the code which can be unambiguously improved across all dimensions. (that is, those lints that we probably want to lift eventually into the compiler).

I know that lint categories exists, but:

• they seem to slice the lints across the different axis -- not the FPR range, but the, well, category
• it's unclear how to actually make clippy run only specific category from the command line.