Elm local install support (#915)

* Add Elm support for npm local installation

test/test_elm_executable_detection.vader

@@ -0,0 +1,36 @@
+Before:
+  call ale#test#SetDirectory('/testplugin/test')
+  runtime ale_linters/elm/make.vim
+
+After:
+  unlet! g:ale_elm_make_use_global
+  unlet! g:ale_elm_make_executable
+
+  call ale#test#RestoreDirectory()
+
+Execute(should get valid executable with default params):
+  call ale#test#SetFilename('elm-test-files/app/testfile.elm')
+
+  AssertEqual
+  \ g:dir . '/elm-test-files/app/node_modules/.bin/elm-make',
+  \ ale_linters#elm#make#GetExecutable(bufnr(''))
+
+Execute(should get valid executable with 'use_global' params):
+  let g:ale_elm_make_use_global = 1
+
+  call ale#test#SetFilename('elm-test-files/app/testfile.elm')
+
+  AssertEqual
+  \ 'elm-make',
+  \ ale_linters#elm#make#GetExecutable(bufnr(''))
+
+Execute(should get valid executable with 'use_global' and 'executable' params):
+  let g:ale_elm_make_executable = 'other-elm-make'
+  let g:ale_elm_make_use_global = 1
+
+  call ale#test#SetFilename('elm-test-files/app/testfile.elm')
+
+  AssertEqual
+  \ 'other-elm-make',
+  \ ale_linters#elm#make#GetExecutable(bufnr(''))
+