Document how to debug this plugin
Fix errors caused by links that contain '%' characters

This error caused the linkify hook to exit prematurely, without
generating any output. The symptom was that the "*ERROR*: Not enough
arguments for format string" appeared on stderr and the rendered page
appeared empty.

Unfortunately, it is not possible to detect this kind of errors from
emacsclient status code. It is always zero.
Set coding system for temporary buffer

Temporary emacs buffers used during processing have not specified a
coding system to use. When debugging this plugin interactively, emacs
asks which coding system it should use for saving the buffer into the
file. This change suppresses these questions.