Add configuration dictionary support to elixir-ls

This adds generic configuration dictionary support to the elixir-ls
linter. This is useful for disabling its built-in Dialyzer support, for
example, which can improve startup time.

The configuration dictionary is a little verbose. I considered reducing
the user configuration to only the nested settings dictionary (and
having the linter implementation wrap it in the top-level `elixirLS`
dictionary), but leaving it fully configurable simplifies the code and
removes any assumptions about current or future ElixirLS behavior.

test/command_callback/test_elixir_ls_command_callbacks.vader

@@ -28,3 +28,8 @@ Execute(should set correct LSP values):
   AssertLSPOptions {}
   AssertLSPConfig {}
   AssertLSPProject ale#path#Simplify(g:dir . '/mix_paths/wrapped_project')
+
+Execute(should accept configuration settings):
+  AssertLSPConfig {}
+  let b:ale_elixir_elixir_ls_config = {'elixirLS': {'dialyzerEnabled': v:false}}
+  AssertLSPConfig {'elixirLS': {'dialyzerEnabled': v:false}}