Added options (-m and -a) on feature finish action that allow to change default merge message.

Those are:
- custom merge message option (-m)
- custom appended message to a default merge message (-a)
- default addition to -a option via git config option gitflow.feature.finish.append.
Keep feature branch when --squash option is used.

Since no merge has happened, feature branch can't be deleted cleanly.
Fix formatting multiline merge message.
Fix bug with incorrect git merge call when $OPTS is empty. Refactor variable names to upper case.