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.

ale_linters/elixir/elixir_ls.vim

@@ -0,0 +1,19 @@
+" Author: Jon Parise <jon@indelible.org>
+" Description: elixir-ls integration (https://github.com/JakeBecker/elixir-ls)
+
+call ale#Set('elixir_elixir_ls_release', 'elixir-ls')
+
+function! ale_linters#elixir#elixir_ls#GetExecutable(buffer) abort
+    let l:dir = ale#path#Simplify(ale#Var(a:buffer, 'elixir_elixir_ls_release'))
+    let l:cmd = ale#Has('win32') ? '\language_server.bat' : '/language_server.sh'
+
+    return l:dir . l:cmd
+endfunction
+
+call ale#linter#Define('elixir', {
+\   'name': 'elixir-ls',
+\   'lsp': 'stdio',
+\   'executable_callback': 'ale_linters#elixir#elixir_ls#GetExecutable',
+\   'command_callback': 'ale_linters#elixir#elixir_ls#GetExecutable',
+\   'project_root_callback': 'ale#handlers#elixir#FindMixProjectRoot',
+\})