#8 Add resource collector plugins

[Szasz Pal]

Resource collector might need to take special actions
to create some files. The most common case is when a
lot of small images are packed in one bigger file. The
resource collector should be able to somehow execute
this external (or maybe internal) programs.

Since the resource files might change names, to save
build time it might be usefull to add the special
actions as plugins.

To make the fewest amount of changes to the code to
support it, here is a proposed syntax:

1) to start a datafile block, the resource file must be
marked with '!', and no alternative name is given
2) to end a datafile block, the resource file must be
marked with '!' and as the original name the plugin
command must be specified

Example: suppose we want to add icon.png directly, but
cursor.png and menubg.png should be packed in gfx.dat:

----8<----
+icon.png
!+gfx.dat
+cursor.png
+menubg.png=menubg-$(vendor).png
!+gfx.dat=jpack gfx.h
----8<----