When the `--extract-plist` option was added to livecheck, conditions
were added in `#run_checks` to skip casks using `ExtractPlist` if the
`--extract-plist` isn't used and the run involves multiple
formulae/casks. This integrates the skip into the `SkipConditions`
class.
The `check_for_resources` code arguably speaks for itself, so it
doesn't feel like a necessary addition. Besides that, this cleans up
some other comments and text to better align with existing examples.
