Feature/aliases

This makes it possible to add keyword aliases.

Here is how you would add the keyword `play` as an alias for `playsong`:
*  Duplicate script filter in Alfred (eg. `playsong`)
* Change the keyword to `play`
* Add the following line to the top of the filter: 
```
-- @ALIAS@ playsong
```
* Run `update-workflow.py`

`update-workflow.py` will update aliases by keeping the first line and appending the contents of the file the keyword is an alias for.