Create a process for snapshotting items

[cdchapman]

More general than issue #43.

• Create Nanoc command to snapshot an item, copying the current state of the file to a new location, possibly in same directory with a Date or Time appended to the filename.
• Create Nanoc rules to process these snapshots special.
• Published items should have a new snapshot each time it changes after publication. , possibly implement a git hook? No, because I want the process to not be automatic to allow for trivial changes and updates without creating a new version.
• Permalinks should link to a representation of the current snapshot of the item, even though the latest version of the resource may be located at a different URL.
• Versioned items should provide a list of historical versions, in reverse chronological order.

Check https://www.w3.org/Provider/Style/URI and the W3C process for versioning specs.