Add Elm linting via elm-make (#213)

* Add support for Elm linting

* Adding documentation for Elm

* Adjusting spacing

* Addressing concerns listed in pull request

Removed the s:FindRootDirectory function as it does not make much sense
in this context. Adjusted the rest of the code to handle the removal of
that function, including using the ale#util function to find the nearest
file.

Ensured that when an empty filepath is found, the code does not attempt
to change directories.

Ensured that the linter would take from stdin using the wrapper.

doc/ale.txt

@@ -71,6 +71,7 @@ The following languages and tools are supported.
 * Cython (pyrex filetype): 'cython'
 * D: 'dmd'
 * Elixir: 'credo'
+* Elm: 'elm-make'
 * Fortran: 'gcc'
 * Go: 'gofmt -e', 'go vet', 'golint'
 * Haskell: 'ghc', 'hlint'