Add the ability to ignore weeds

[ocharles]

Fixes #56

[ocharles]

This is currently in limbo pending support from users. I thought I needed this, turns out I don't. But maybe others do?

[EncodePanda]

I've "just" started using weeder for my personal projects, so I don't know yet :)

[shapr]

Work codebase is 500kloc and we have several example projects that aren't roots and I would like to ignore.
I've set them as roots, but I could see the benefit of ignoring Example.hs and that sort of thing!

[anka-213]

Yes, this feature would be useful.

[philderbeast]

For haskell/cabal, it might help there being able to ignore cabal-testsuite:

$ weeder --version
weeder version 2.7.0
hie version 9063
$ weeder --write-default-config
Did not find config: wrote default config to ./weeder.toml
incompatible hie file: /.../cabal-testsuite/PackageTests/CopyHie/setup.dist/work/dist/build/extra-compilation-artifacts/hie/HieLocal.hie
    this version of weeder was compiled with GHC version 9063
    the hie files in this project were generated with GHC version 9047
    weeder must be built with the same GHC version as the project it is used on

[NorfairKing]

@ocharles I definitely need this for false positives such as TH splices.

In fact, the last three times I tried using weeder, I decided not to because I couldn't ignore false-positives :P
Now I figured out that you can work around this by adding false positives as roots, but then those grow weeds as well, so requiring the ignores to match something is necessary too.