Introduce a mechanism in this source tree for building the container

web pages for the Java applets. Previously, those have all been
maintained by hand in my website's svn area, which is a bit silly. Now
we have a file per puzzle in the 'html' subdirectory which contains
the puzzle's name, one or two attributes, and the instructions snippet
to go below the puzzle applet; and then there's a Perl script that
builds all the real web pages out of that by adding in the parts
common across all files: the header, footer, and middle fragment with
the <applet> tag and resizing bits and pieces.

One piece _not_ checked in here is the footer text specific to my
hosting at chiark, which I think does still belong in the www area. So
Buildscr doesn't actually build the web pages; it just delivers the
bits and pieces by which my nightly snapshot script will be able to
run the program that _does_ build them, passing that footer as an
extra argument.

[originally from svn r9780]

html/cube.html

@@ -0,0 +1,14 @@
+Cube
+<p>
+Roll the cube around the grid, picking up the blue squares on its
+faces. Try to get all the blue squares on to the object at the same
+time, in as few moves as possible.
+<p>
+Use the arrow keys to roll the cube, or click the mouse where you
+want it to roll towards. After every roll, the grid square and cube
+face that you brought into contact swap their colours, so that a
+non-blue cube face can pick up a blue square, but a blue face rolled
+on to a non-blue square puts it down again.
+<p>
+When you have mastered the cube, use the Type menu to select other
+regular solids!