Support specifying a number of hosts between input

Just a minor thing I've wanted, since I usually find myself doing "c" 5-10 times at the start of the push to get the new code out to a few hosts and make sure things don't explode before letting the push loose. This allows you to either do something like `--numhosts 5` in the push command to go to 5 hosts initially, or you can also enter a 1-9 number at the CLI input where you could previously do `a` / `c` / `x`.

I think this should be correct, though I'm not really sure how to test the push tool. One slight implementation oddity is that in the case of an auto-continuing push, the `hosts_before_pause` variable is going to go into the negative. I could specifically prevent that, but it didn't seem very important.

:eyeglasses: @spladug 
Does this make `0` an alias of `a`? Is that intended?