Add options for strategy and merge-strategy

[deiga]

For hotfix merges I'd like to use git merge -s recursive -X ours so it would be great if gitflow would accept the -s and -X options :)

[sishbi]

I would also like this option, for release and feature branches...

[sirolf2009]

Same here. We use release to track when and what code we push to prod, which is automated by jenkins because our environment consists of about 30 projects. Merging them all manually is hard and the lack of an automatic merge strategy kind of breaks the jenkins workflow

[sirolf2009]

https://github.com/petervanderdoes/gitflow-avh/pull/386