"Rebase" and "Finish" support for features based on branches other than "develop" (for example other features).

More details about this PR: https://github.com/shpoont/gitflow/wiki/Working-with-features-based-on-branches-other-than-%22develop%22.

There will be another PR which will support remotes for similar work flows.

Wiki changes:  
 - https://github.com/shpoont/gitflow/wiki/Reference:-Configuration/_compare/05f440725107e396798c0cbb0ad4a3fa4ed83718...cb05d51f7b88f8ef004f3e238cf59f9db77486ee  
 - https://github.com/shpoont/gitflow/wiki/Reference:-git-flow-feature/_compare/0aed0135470ad40ff1deb60cc9b698c5cc2e2234...fa865f85538409f173635b2431d057f2adeeb500