support multiple paths for a browser

Allow multiple paths to be specified for a single browser. This change will select the first path that exists.  Example:

```javascript
    "command": [
        "c:/Program Files/Mozilla FireFox/firefox.exe",
        "c:/Program Files (x86)/Mozilla Firefox/firefox.exe"
    ]
```
This can be useful when running a portable installation of Sublime and the configuation is shared (for example, on DropBox) and different machines have browsers in different locations.