Implement Issue #245 Snapshot exclude patterns

[exi]

Issue #245

This is mostly a first draft without tests, but it seems to work and i'm using these changes on top of v3.2.2 on my server.

The primary point why I needed this is because in the time zfs_autobackup runs, my zfs-autosnapshot has taken and removed multiple "frequent" snapshots, which leads to autobackup failures. I just want autobackup the snapshots with "monthly", "weekly" and "daily" in the name.

[exi]

@psy0rz I just wanted to get early feedback if this approach seems reasonable. I know you don't have much time to implement it yourself, so worst case i'll just use my own patched version on my server for the time being.

[psy0rz]

looks good, but "is_exluded" is too generic a name i think?

[psy0rz]

uh oohh some tests failed?

[psy0rz]

oh nevermind they were already broken, i'll have to fix that first

[psy0rz]

i fixed all the tests, can you rebase on master so that we can run the tests?

[exi]

@psy0rz rebased ✅

[coveralls]

coverage: 86.145% (-0.3%) from 86.462% when pulling 67f6f37a8fd99d4fe2f596913daf69ce5e6cd38b on exi:snapshot_exclude_245 into 9e2476ac840efb55ae21d5126483b0dbb6a07c15 on psy0rz:master.

[psy0rz]

thanks!