Add cleanup CLI

[martialblog]

• Adds a simple command to cleanup older backup files. Intention was to provide something users can use in a cronjob/timer.

# Removes all but one backups for all views
icingacli toplevelview
# Removes all but one backups for a specific view
icingacli toplevelview --view myview
# Removes all but two backups for a specific view
icingacli toplevelview --view myview --keep 2

Fixes #71