Don't load themes and ext. plugins on "dumb" terminals

Don't load themes and ext. plugins on "dumb" terminals

These external (and modern?) plugins most probably don't expect such an
unusual old terminal configuration and don't behave well (echo color
sequences, for example). Therefore we DON'T load any external plugins at
all when running on such a terminal: this results in reduced/disabled
functionality, but hopefully in readable output ...

And the same is true for any themes, included or external: we disable
them and fallback to an included very simple but sane default prompt.