Add git's global gitignore setting

Issue solved: Respect git's global gitignore setting.

Solution: Execute git command to check if there's a global gitignore
setting.  If so, use that as a further filter for ignoring files.

- Add unique option for NoGlobalGitIgnore
- Option defaults to `false`
- If option is false, query git for globalignore path.

Credit for idea and git commands:
https://github.com/ggreer/the_silver_searcher/pull/175.
Merge pull request #42 from zph/master

Add git's global gitignore setting