Adding $.format

Added `$.format` function which allows formatting strings using placeholders.

```
var str = $.format('This is my {0}. Do you like {1}?', 'string', 'it');
// produces 'This is my string. Do you like it?'
```

Status: **Ready to merge**

Reviewers: @tedtate @ericmuyser 

## Changes
- Added format.js file
- Added tests

## How to test-drive this PR
- checkout branch
- `grunt test`