Improve heatmap visualization options (more color schemes, better readability)

[nfelnlp]

For once, there could be an option to choose different colors instead of red and blue as inspired by this site.

https://github.com/nfelnlp/thermostat/blob/b7f8e829ca2927121c7fa24eb977ecb91a92d017/src/thermostat/visualize.py#L46

On top of that, the current color scheme is not perfect for readability, because some of the most salient words (very red or very blue) still have black text and cannot be read properly. Maybe it would be better to have a colored border around the word instead?

https://github.com/nfelnlp/thermostat/blob/b7f8e829ca2927121c7fa24eb977ecb91a92d017/src/thermostat/visualize.py#L147-L152