Option for adding files to gitignore

We should add an option which automatically adds some items to the gitignore beside the items in `.gitignore` eg: `gitfs git@github.com:PressLabs/gitfs gitfs -o ignore=*.swp -o ignore=*.bak`.

Also we can add an option for reading an additional gitignore-like file:
`gitfs git@github.com:PressLabs/gitfs gitfs -o ignore_file=/etc/gitfs.ignore`
