~jcc/addr-book-combine

Add contributing info
Print usage without logging info (e.g. timestamp)
Don't fail when command exits with non-zero status

Some commands, like khard, exit with a failure status when no entries
are found. No results is fine, and other address books may have entries
that still need to be displayed. Print a warning log instead.
Preserve original case of address

Address de-duplication is still case-insensitive.
Remove unneeded trailing tab
Compare addresses case-insensitively
Reference maildir-rank-addr
Initial working version