Add elixir-ls language server support

ElixirLS (https://github.com/JakeBecker/elixir-ls) is an LSP server for
Elixir. It's distributed as a release package that can be downloaded
from https://github.com/JakeBecker/elixir-ls/releases or built locally.

The easiest way to start it is via Unix- and Win32-specific helper
scripts, so that's the basis of this command integration. Alternatively,
we could implement the contents of those platform-specific scripts in
the linter's command callback in a language-neutral way, but there isn't
any benefit to doing that aside from eliminating the platform check, and
that could prove to be too tight of a coupling going forward.

test/command_callback/test_elixir_ls_command_callbacks.vader

@@ -0,0 +1,29 @@
+Before:
+  call ale#assert#SetUpLinterTest('elixir', 'elixir_ls')
+
+  let g:ale_has_override['win32'] = 0
+
+After:
+  let g:ale_has_override = {}
+
+  call ale#assert#TearDownLinterTest()
+
+Execute(should set correct defaults (unix)):
+  AssertLinter 'elixir-ls/language_server.sh', 'elixir-ls/language_server.sh'
+
+Execute(should set correct defaults (win32)):
+  let g:ale_has_override['win32'] = 1
+
+  AssertLinter 'elixir-ls\language_server.bat', 'elixir-ls\language_server.bat'
+
+Execute(should configure elixir-ls release location):
+  let b:ale_elixir_elixir_ls_release = 'boo'
+
+  AssertLinter 'boo/language_server.sh', 'boo/language_server.sh'
+
+Execute(should set correct LSP values):
+  call ale#test#SetFilename('mix_paths/wrapped_project/lib/app.ex')
+
+  AssertLSPLanguage 'elixir'
+  AssertLSPOptions {}
+  AssertLSPProject ale#path#Simplify(g:dir . '/mix_paths/wrapped_project')