Add another fuzzy finders support

[a5ob7r]

Fixes #10

Different points from it is below.

"${FZPAC_FINDER}"'s value is colon separated list like a "${PATH}" variable, elements are but commands not directories. fzpac trys to use each of commands in "${FZPAC_FINDER}" in turn as fuzzy finder to find packages. So fallback to following commands if preceding one isn't found.

If no "${FZPAC_FINDER}" is set or empty, fzpac assumes the value is 'fzf:sk:peco:gof:fzy'.

# Elements are command name or path.
FZPAC_FINDER="finder1:/path/to/finder2:..." fzpac ...

# Precede your favorite fuzzy finder if you want to give priority to it.
FZPAC_FINDER="sk:fzf:..." fzpac ...

[sheepla]

Thank you for your PR!🙏 I confirmed the operation. I will merge it!