Option for global hooks and filters directory (Fixes #82)

Summary
* Default hooks folder is .git/hooks
* It can be configured by setting "gitflow.path.hooks" in the config, per-repo, per-user or system-wide. e.g. `git config --global gitflow.path.hooks /usr/local/bin/gitflow-hooks`
* Removed extra "flow" in hook and filter filenames

**Please do not forget to update wiki**
https://github.com/petervanderdoes/gitflow/wiki/Reference:-Hooks-and-Filters