Add web view -> cocoa bridge

This adds an object to ```window``` which makes use of the cocoa web view windowScriptObject.
It allows (currently) for bash commands to be ran via javascript like so:
```javascript
var output = window.flashlight.bash('whoami');
// 'output' now equals current username
```
The included bash method is just a very small function made for a PoC. However, this allows full javascript to cocoa/obj-c interaction and will change plugin functionality completely.

Fixes [#117](https://github.com/nate-parrott/Flashlight/issues/117)