add --draft/-d option

i considered making --draft the default. however, i didn't like the
change of default behavior.

then it occurred to me that automatically detecting WIP subjects like
the sanity bot does would actually fit our current workflow pretty
neatly.
we are looking only at the last commit, as that's likely to be the one
which is WIP. also, in "unbased" push mode, determining the start of the
range to look at would cause quite some overhead.

Change-Id: I70f6f397693f600b6a5d5791f04d9cae12fa8d02
Reviewed-by: Sergio Ahumada <sahumada@blackberry.com>