Change CMS_CASCADE_LEAF_PLUGINS to a dictionary for more configurable settings

See #5 and #6.

Putting default outside of any framework so no need to specify multiple default settings.
```
CMS_CASCADE_LEAF_PLUGINS = {
  'bootstrap3': {
    'SlidePlugin': ('FilerImagePlugin',)
  },
  'default': ('TextPlugin', 'FilerImagePlugin',),
}
```