[[!template  id=gitbranch branch=chrismgray/exclusive-hooks author="[[chrismgray]]"]]

Sometimes plugins register a function with `hook`, but they only want
the function called with the content that they know how to deal with.
Normally, this means that they call `pagetype` first thing in the
function, determine if they know how to deal with the content, and
only do anything if they do.  

This is a bit wasteful in itself, but for external plugins, it's
really bad.  For functions like `scan` and `linkify`, where the entire
page is sent back and forth over `stdout` and `stdin`, it really slows
things down.  

Thus, I propose that there be a new optional parameter to `hook` that
tells it that the function should only be called for files whose type
is the same as the id of the plugin calling `hook`.  I have called
this parameter `exclusive` in my branch, but this might not be the
best name.

[[!tag patch]]