#52 Add SET ARCHIVE command

[LesK]

Something like: SET ARCHIVE ON/OFF [[suff nn [filespec]]]
Much like BACKUP except the suffix is incremented for each edit
session to a max value of NN (without replacing the previous version), with 1 always being the most recent archive copy and THE automatically renaming older copies.

For example: The first time a file is saved, the previous version would
have a suffix of .arch1
Then some number of days later when the file is edited again and
saved, .arch1 gets renamed to .arch2 and the new archive copy
is saved as .arch1

Filespec should be designed to allow the user to specify where the
archive is to be kept and a prefix for the filename, perhaps triggered
by a keyword that THE would handle, like 'datetime'.