Improve performance

[puntonim]

Performance could be improved if the configuration were hard-coded in promptastic.py. Currently promptastic reads the configuration in config.py to display the enabled segments with the chosen theme's colors. This could be done better this way:

• promptastic.py becomes a template, say promptastic.py.template
• install.py creates the actual promptastic.py script out of that template by reading config.py and injecting only the enabled segments with hard-coded colors.