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/keen.html

@@ -0,0 +1,15 @@
+Keen
+<p>
+Fill in the grid with digits from 1 to the grid size, so that every
+digit appears exactly once in each row and column, and so that all
+the arithmetic clues are satisfied (i.e. the clue number in each
+thick box should be possible to construct from the digits in the box
+using the specified arithmetic operation).
+<p>
+To place a number, click in a square to select it, then type the
+number on the keyboard. To erase a number, click to select a square
+and then press Backspace.
+<p>
+Right-click in a square and then type a number to add or remove the
+number as a pencil mark, indicating numbers that you think
+<em>might</em> go in that square.