Silence the call to 'which' in the pre-commit hook.

It doesn't spew out errors when it doesn't find the executables anymore.
Add force install option.

Useful for evil unattended forced installs.
Add initial support for ruby code style checking using the rubocop gem.
Add documentation for rubocop check.
Add config file setting to rubocop check.

And document it.
rubocop_check: don't pass the config file argument if file does not exist.
Merge branch 'rubocop_check'