Feature request: Add option to add up -m and -M minimal available memory

[mars4science]

If one includes the app in the distro, it is IMO useful to have universal /etc/default/earlyoom.

IMO it is more "universal" to have minimal memory at fixed amount+percentage. To keep the app backward compatible, please add option to add up -m and -M instead of choosing min of them. Thanks.

Edit: created pull request https://github.com/rfjakob/earlyoom/pull/305 with this feature. Please see documentation and code, review and merge.