Add hook for custom search commands.

Created a hook for add-ons to add custom search commands to the browser. An add-on creates a new command (e.g. kanji-grade) with the following code:

```python
def onSearch(cmds):
     cmds['kanji-grade'] = findKanjiByGrade

def findKanjiByGrade((val, args):
     <write code here filter results>
     
addHook("search", onSearch)
```