* Add better support for Haskell stack compiler tools
This commit adds support for `stack` as the executable of a tool. This
follows a pattern that has been implemented for `bundler`'s tool chain.
* Move hlint command to linter file
* Add vader test for stack exec handling
* Update ghc-mod to support stack execution
`ghc-mod` was previously broken into 2 linters.
1. ghc_mod
2. stack_ghc_mod
This additional linter is not necessary with proper support for
executable variables and `stack exec` handling.
* Support stack exec in hfmt
* Support stack in hdevtools
