Add snippets/completions for main / source file paths

Consider the following interface:


```
bower<tab>
```

Should autocomplete / list the available packages that you have installed in a contextual menu (not sure if possible? - need to research).
Pressing `<enter>` should complete the string into `bower:packageName`

```
bower:packageName<tab>
```

Should insert the paths(s) of the main files outlined in the packages' `component.json` file.
If you're currently focused on a HTML file, it should:

* Write `<script src="components/jquery/jquery.js"></script>` for scripts
* Write `<link rel="stylesheet" href="comonents/normalize-css/normalize.css">` for stylesheets

If you're currently focused on any other kind of file, paste the raw path. 

If anyone wants to start implementing this, that'd be gnarly. Otherwise, I'll roll my sleeves up and sort it out when I get the chance.