Add ability to "give a raise"

[TomAFrench]

We could give the stream sender the unilateral ability to bring forward the stream's end time.

This would in essence increase the rate of the money being streamed, i.e. a raise in the example of a salary.

Alternatively we could allow the sender to add additional funds without pushing out the stream's end time. (possibly configurable on a sliding scale with #30?)