Automatic swatch discovery

- Add a `getSwatches()` function that can find swatches programmatically
- Implement a test for `getSwatches()` (note that this will either need to be
  updated or removed when the file structure of the repo changes)
- Generate the config for the swatch task programmatically
- Add a `swatches` task to list all themes

Note that `getSwatches()` is a little fragile, as it will have to be updated if any non-swatch directories are added to the repo. It may be a better idea to move all swatches into one directory to remove the need for a blacklist, though that could break projects depending on Bootswatch.