Extend snapshotcontent.py to use optional date and object type filters

[TomArne]

An optional parameter of date filter (on modified and/or create datetime) as well as content type filter would allow for implementation of incremental backup from of SAS Content - allowing partial restoring of SAS Content that for instance have been mistakenly deleted or mistakenly overwritten.

A full snapshot every night adds up in both running time and space needed to store export packages.
A monthly or weekly full snapshot followed by daily incremental snapshots would be much more efficient.

[gerrynelson63]

@TomArne please check out snapshotcontent.py in the branch dev-snapshotdate and let me know what you think.

On the content type filter do you think it should be a list of types to include or a list of types to exclude? I am undecided about which would be easiest for the user.

[gerrynelson63]

Re-opened to cover request to be able to subset by type.