Add Monokai theme

This pull requests adds a Monokai theme to PlainTasks.  It is based specifically on the Monokai Soda theme vs. the default Monokai.  The only real difference is that the background is a little darker to provide greater contrast. 

It looks great though!

![3f4e](https://cloud.githubusercontent.com/assets/259316/4128602/4f3da0ba-330e-11e4-9c1a-c4e6eb945440.png)

To use it just add

```json
{
    "color_scheme": "Packages/PlainTasks/tasks-monokai.hidden-tmTheme"
}
```

To your `PlainTasks.sublime-settings` file.