[FR] Ignore a given package

When CompatHelper is used for subdirs, e.g. `docs` or eventual `examples`, one wants to use the latest commit version of the current PR or of master/main from the parent repo/package.
Therefore putting a `compat` for the parent package does not makes sense. A solution would be to pass a list of packages to ignore when running `CompatHelper.jl`, maybe a `blacklist` keyword?