Add syntax highlight to mix.lock files (#105)

Author: AlternateRT

languages/elixir/config.toml

@@ -1,6 +1,6 @@
 name = "Elixir"
 grammar = "elixir"
-path_suffixes = ["ex", "exs"]
+path_suffixes = ["ex", "exs", "mix.lock"]
 line_comments = ["# "]
 autoclose_before = ";:.,=}])>"
 brackets = [