For a hook client-detached with many `rm cachedir/pid`'s every one is
run, no matter which client is detached. This commit adds a conditional
to check if the "concerned" client exists. If it does not, then only it
removes the cache.

BREAKING: style strings now take `@[]` instead of `#<>`
feat:     Format strings for --after and --before now take any number of
          style strings at any positions.
feat:     New --width option for specifying min width of usage string