`lint_file` can now be computed dynamically with a callback function,
which can return a deferred result, as per `ale#command#Run`. This
allows linters to dynamically switch between checking files on disk,
or checking code on the fly.
Some tests have been fixed on Windows.
It's common to add SwiftLint as a CocoaPod dependency, instead of as a global
binary. In this case we should use that version of SwiftLint before looking
for any others. Note that I'm also adding support for SwiftLint in ReactNative
projects here as well, where the Pods directory would be nested inside an ios
directory.
