Missing dependency check

[sdx23]

Ideally the plugin would check for dependencies (refactor binary in path, hlint in fitting version) at load and be disabled if they aren't found.

[bjornbm]

And if hlint and refactor are not found globally try with, e.g., stack exec hlint as they might be project-local (with stack build) or compiler-specific (with stack build --copy-compiler-tools).