Add an option sell_min_amt

[firepol]

Around 2 weeks ago @T-rav added this option (see #1051):
--buy_max_amt buy with up to this amount of currency balance

It would be nice to have the opposite option (for selling):

--sell_min_amt sell with at least this amount of currency of asset

This would be useful to avoid selling too low amounts, especially if sell_pct is set to a value less than 100%.

[firepol]

@infatrus this a specific feature needed for zenbot to be coherent with the buy_max_amt option.

Other issues you are experiencing: if I were you I'd file separate bug reports/feauture requests for them, else we'd mix too much stuff in one thread if you see what I mean.