Grunt clear bootswatch option

Added a swatch option to clear bootswatch discussed a little while ago in #145. Initially I implemented the option as `--reset` but felt that this gave the expectation that the command would reset the theme to it's distribution state which uses Cerulean vars. `--none` seemed clearer that the swatch is completely removed so that afterwards the theme shows vanilla Bootstrap.

```
% grunt swatch --none
```