allow using any dmenu-like backend

`tessen` now allows using any wayland native dmenu like backend. To
retain backward compatibility, the default options for bemenu, rofi, and
wofi are still provided. In addition, default options for fuzzel were
added. The complete list of changes are:

802a640 release: revamp the README, bump version of tessen
06d877f refactor: change the order of dmenu backend
5c6b52d docs: update the man page
bcc0a07 style: use consistent case, fix grammar
ed9412a refactor: use only one printf
ca9d486 refactor: change the find_backend function
edea7a8 refactor: change the setup_backend function
fc86aa3 refactor: simplify the default opts
f3b3176 refactor: remove redundant colons
2e4d8ee allow using any dmenu-like backend
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