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

Add license

Initial working version