User defined colour palettes

I have looked for similar feature requests, but I couldn't find any. In other terminals, you can install extra colour palettes in a predefined config folder. For instance, konsole has a `.colorscheme` file type that when deployed under `$XDG_DATA_HOME/konsole/` will appear in the selection menu.

I would like to implement something like this for guake. I could extend `palletes.py` to look for theme files in a directory and import them. I would suggest a JSON structure that resembles the internal structure of `PALETTES` in `guake/palettes.py`, something like:

```json
{
"": ""
}
```

For instance, I love snazzy for konsole, and I ported it manually, but I would like to have a file like `~/.config/guake/themes/snazzy.json` with the configuration, so that I could play around with other schemes and go back to my custom scheme without losing it.

I can open a prototypical pull request to illustrate what I want, if that is of interest.

Thanks!