Bind key to toggle through selectable themes in rtorrent config

[chros73]

Bind key | to toggle through selectable themes.

• include all the 256 color themes from rtorrent-ps project (https://github.com/pyroscope/rtorrent-ps/tree/master/docs/examples)
• include a general pyro.import method with which we can easily import config files from ~/.pyroscope dir
• add couple of methods that contains the available/desirable themes, e.g. cfg.theme.0
• use pyro.theme.selected string variable method to store the actual filename is being used
• apply toggle logic in form of using branches (since we can't write recursive methods in rtorrent config)

Note: the latter means that if a new theme is added to /removed from the available selection then the toggle logic has to be modified as well.

Use partly 0.9.6 coding style.

Important: this change didn't update the setup scripts that should copy the new color config files into ~/.pyroscope dir.