Eskil provides a plugin system where a plugin can preprocess data
before being compared and displayed.

The command line options for plugins are:
  -plugin plugin     : Use plugin
  -plugininfo info   : Pass info to plugin (plugin specific)
  -plugindump plugin : Dump plugin source to stdout
  -pluginlist        : List known plugins
  -pluginallow       : Allow full access for a plugin.
A plugin may further define command line options that it accepts.

A plugin is a Tcl script that must follow a specific format.
Dump one of the included plugins to see what it looks like.
The plugin is executed in a safe interpreter and thus cannot do any
damage. You can turn this safety off with -pluginallow.

A plugin is set up with these global variables filled in:
::WhoAmI  : The name of the plugin
::Info    : The contents of -plugininfo parameter
::Pref    : A copy if Eskil's internal preferences array.
::argv    : A copy of the command line from Eskil's invocation

Example plugins are included in the kit.

A plugin may give a result that has a line-by-line correspondence to
the original, in which case the preprocessed data is used for comparing
while the original is used for displaying.  The main plugin procedure
returns 0 to signify this case.

If the plugin procedure returns 1, the processed data is used also for
displaying.

When searching for a plugin "x", files "x" and "x.tcl" will
match. The search path is current directory, "plugins" directory,
the directory where Eskil is installed, "plugins" directory where
Eskil is installed, and also the internal "plugins" wrapped into Eskil.