Use a variable for the purge command

Reduce risk of having a discrepancy between the displayed command
and the actual purge command.
Added -q option although I prefer -qq so there is no prompt. But
I also want to see what's being removed in case of some broken
dependency that tries to uninstall half the system (most likely
happen in a dev release)
Add quiet mode; specify # kernels to keep

Still have default of keeping 2 kernels
Allow user to keep 1 to 9 with --keep= option
Quiet mode doesn't prompt (except to enter password
for elevation of privileges (sudo) if required
Use variable for autoremove/autoclean commands