Option for global hooks and filters directory fix

Summary
* `git flow init` will ask for hooks directory path
* Default hooks directory is .git/hooks
* It can be configured manually 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`
* Added global variable "GIT_CURRENT_REPO_DIR" containing absolute path to current git repository
* Removed extra "flow" in hook and filter filenames

**Please do not forget to update the wiki**
* https://github.com/petervanderdoes/gitflow/wiki/Reference:-Hooks-and-Filters
* https://github.com/petervanderdoes/gitflow/wiki/Reference:-Configuration