User-defined resources for the  --watch option

This will allow compilation of files other than .js, .css, and .json files using the shifter --watch command. Basically, I want to watch for any changes in LESS files so that the a module's CSS is updated automatically.

This command will commonly be used in conjunction with the "exec" option in a module's build.json.

An example build.json:
```javascript
{
    "name": "module",
    "exec": [
        "less-compile"
    ],
    "builds": {
        "module": {
            "jsfiles": ["module.js"],
            "skinnable": true
        }
    }
}
```

### How to use:
This can be added to the .shifter.json file:
```javascript
{
    "watched-extensions" : {
        ".less": true,
        ".js": true,
        ".json": true
    }
}
```

##### or alternatively,
shifter --watch js --watch less --watch json

### Known issues:
Currently, CSS and LESS files cannot be watched together else shifter will indicate that the CSS file has been changed too quickly. This is why the previous example does not watch CSS, only LESS.