Progress bar and spinner widgets should allow being driven manually

[dfunckt]

The fact that each instance of these widgets runs each own timer and assumes output is to stdout limits their usefulness a lot. It’d be very helpful for these widgets to provide a way to get the plain rendered widget as a string, without escapes codes for replacing lines, etc. They would then be able to be used in larger compositions of widgets that are driven by a common timer and possibly piped to intermediate streams before reaching stdout.

This ties with #45 and possibly supersedes it.