parse_timespan: add support for an "abbreviated" output

[sandrotosi]

In [52]: humanfriendly.format_timespan(2576)
Out[52]: '42 minutes and 56 seconds'

it would be great if an abbreviated=True argument was present to produce:

In [52]: humanfriendly.format_timespan(2576, abbreviated=True)
Out[52]: '42m 56s'

parse_timespan accepts that shortened version already.

thanks!

[fsaintjacques]

That would be format_timestamp. I'd like to have this too.