Support $VCSH_GITIGNORE == none

This will neither add core.excludesfile nor write a ignore file.
Simplify example setup

Git supports the push -u flag since a while now,
and it's nice to show the user that afterwards
one can push without arguments
README.md: Fix mistake
CONTRIBUTORS: Add Thomas Ferris Nicolaisen
debian/copyright: Update year information
Add pre/post-clone hooks
Complete on git subcommands
Update copyright date in manpage
Add support for `vcsh pull`

This pulls all vcsh repos one after the other.
Add support for `vcsh push`

This pushes to the respective remotes of all vcsh repos.
Add retire() as a complement to use()
Add hook post-clone-cleared
doc/hooks: Update
doc/hooks: New format to make merge easier
vcsh: Formatting
Merge branch 'master' of github.com:RichiH/vcsh
vcsh: Fix `vcsh rename`

Based on report and work by Alexander Skurikhin.

GitHub: fixes 54
vcsh: Fix stupid syntax error

Thanks to Alessandro Ghedini.
CONTRIBUTORS: Add Alexander Skurikhin
Merge branch 'master' of github.com:RichiH/vcsh