Use upstream's location for configuration file

[wesbarnett]

Other distributions follow upstream's location of /etc/sysconfig/snapper. Also add an installation flag of some sort to change this location (e.g., for Arch it would be /etc/conf.d/snapper). Then for Arch the flag could be used with the AUR package.

cf. #42

[rralf]

EXCLUDE_UUID=$(findmnt -n -v -t btrfs --target $SNAPPER_CONFIG -o UUID)

will fail, if /etc/ is no btrfs (in my case, / is ext4):

Exited due to error on line 167.
exit status: 1
command: EXCLUDE_UUID=$(findmnt -n -v -t btrfs --target $SNAPPER_CONFIG -o UUID)
bash line: 0
function name: main

[wesbarnett]

@rralf Issue #44 opened.