Implement no-dependency system and script hook types. Closes #39.

testing/resources/prints_cwd_repo/hooks.yaml

@@ -1,4 +1,4 @@
 -   id: prints_cwd
     name: Prints Cwd
-    entry: prints_cwd
-    language: python
+    entry: pwd
+    language: system

testing/resources/script_hooks_repo/bin/hook.sh

@@ -0,0 +1,4 @@
+#!/usr/bin/env bash
+
+echo $@
+echo 'Hello World'

testing/resources/script_hooks_repo/hooks.yaml

@@ -0,0 +1,4 @@
+-   id: bash_hook
+    name: Bash hook
+    entry: bin/hook.sh
+    language: script