As is the convention in prezto, we cache the command-not-found handler to
avoid incurring the performance penalty of loading ruby interpreter on
every call. This restores the 'Homebrew way' of loading command-not-found
handler.
Further, the formally recommended command lookup mechanism in Homebrew
(viz., `brew command command-not-found-init`) is ruby based and is super
slow. To avoid performance penalty, we `find` it ourselves from
`TAP_DIRECTORY` defined internally in Homebrew.
This also reinstates support for custom taps or non-standard Homebrew location.
Indrajit Raychaudhuri
Kaleb Elwert
Ashish Gandhi
Sorin Ionescu
Daniel Renninghoff