This adds support for the hdevtools haskell linter
https://github.com/hdevtools/hdevtools
The output for hdevtools is near identical to the ghc output so this
also extracts the ghc handler into the handle file and adds tests
For ghc, it seemed that the conditional
```
if l:corrected_lines[-1] =~# ': error:$'
let l:line = substitute(l:line, '\v^\s+', ' ', '')
endif
```
was never being reached. It's actually better to unconditionally
collapse whitespace anyway and so I simply removed the conditional
check.
For hlint I added more information about the error. This changes the
reported error from `Error:` to something like:
` Error: Avoid lambda. Found: \ x -> foo x Why not: foo`