Use case instead of MatchErrror

Instead of pattern matching causing a MatchError, use case directly
matching pattern for the happy path and for the invalid argument path.

lib/cli.ex

@@ -4,16 +4,12 @@ defmodule Onigumo.CLI do
   }
 
   def main(argv) do
-    try do
-      {[], argv, []} = OptionParser.parse(argv, strict: [])
-      argv
-    rescue
-      MatchError -> usage_message()
-    else
-      [component] ->
+    case OptionParser.parse(argv, strict: []) do
+      {[], [component], []} ->
         {:ok, module} = Map.fetch(@components, String.to_atom(component))
         root_path = File.cwd!()
         module.main(root_path)
+
       _ ->
         usage_message()
     end