tmux-gitbar.conf should be autogenerated on first run

[arl]

The configuration file should be:

• auto-generated on update-gitbar first run (why not during interpreting of tmux.conf as this would not add any workload to update-gitbar)
• out of version control

This would permit to modify the default configuration, and not having it overwritten by tmux-gitbar updates.
Obviously if it already exists it should not be recreated...