feat: allow configuring additional linters to run

[grunweg]

Extend getChecks to allow an additional list of linters, which is always run. This can be used downstream to e.g. enable the docBlameThm or explicitVarsOfIff linters without modifying batteries.

As part of this, we

• update fix the function docstring (which was quite outdated anyway)
• remove the useOnly option: the equivalent effect can be achieved by filtering the list of linters afterwards; adding linters cannot, hence must be performed in this function. (That said, I'm happy to add this functionality back if desired.)
• rename a variable dflt to the more readable default
• adjust the logic of the #lint command accordingly Note that test/lint_simpNF.lean lies on this (the docBlame linter warns there) - so this changed is already tested.

[grunweg]

awaiting-review

[grunweg]

I just pushed a commit adding back the useOnly option as runOnly.