Offer easy way to override FlagUsages' format string

[blixtra]

The format string in the FlagUsages method of FlagSet currently is set like this...

format = format + ": %s\n"

It would be nice to be able to set this to be tab separated instead.

format = format + ": \t%s\n"

It would be nice to be able to set the delimiter somewhere instead of having to override this by reimplementing (copy & pasting) the entire method.

[tomasaschan]

This issue has not been updated in a long time. Is it still relevant?

(If I receive no answer before the end of the summer, I'll close it. Feel free to re-open if you come here after that happens and it is relevant to you - but if you do, please also comment and specify what outcome you're interested in!)