underlay plugin, a command-line interface for add_underlay()

doc/plugins/contrib/underlay.mdwn

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+[[!template id=plugin name=underlay author="[[Simon_McVittie|smcv]]"]]
+[[!tag type/useful]]
+
+This plugin adds an `add_underlays` option to the `.setup` file.
+Its value is a list of underlay directories whose content is added to the wiki.
+
+Multiple underlays are normally set up automatically by other plugins (for
+instance, the smiley images used by [[plugins/smileys]]), but they can also be
+used as a way to pull in external files that you don't want in revision control,
+like photos or software releases.
+
+Directories in `add_underlays` should usually be absolute. If relative, they're
+interpreted as relative to the parent directory of the basewiki underlay, which
+is probably not particularly useful in this context.
+
+    #!/usr/bin/perl
+    package IkiWiki::Plugin::underlay;
+    # Copyright © 2008 Simon McVittie <http://smcv.pseudorandom.co.uk/>
+    # Licensed under the GNU GPL, version 2, or any later version published by the
+    # Free Software Foundation
+
+    use warnings;
+    use strict;
+    use IkiWiki 2.00;
+
+    sub import {
+    	hook(type => "getsetup", id => "underlay",  call => \&getsetup);
+    	hook(type => "checkconfig", id => "underlay", call => \&checkconfig);
+    }
+
+    sub getsetup () {
+    	return
+    		plugin => {
+    			safe => 0,
+    			rebuild => undef,
+    		},
+    		add_underlays => {
+    			type => "string",
+    			default => [],
+    			description => "extra underlay directories to add",
+    			advanced => 1,
+    			safe => 0,
+    			rebuild => 1,
+    		},
+    }
+
+    sub checkconfig () {
+    	return unless exists $config{add_underlays};
+
+    	foreach my $dir (@{$config{add_underlays}}) {
+    		add_underlay($dir);
+    	}
+    }
+
+    1;