Support for loading games in Javascript puzzles.

This is done by showing a dialog containing an <input type="file">
through which the user can 'upload' a save file - though, of course,
the 'upload' doesn't go to any HTTP server, but only into the mind of
the Javascript running in the same browser.

It would be even nicer to support drag-and-drop as an alternative UI
for getting the save file into the browser, but that isn't critical to
getting the first version of this feature out of the door.

html/jspage.pl

@@ -210,6 +210,7 @@ ${unfinishedpara}
 ><li id="specific">Enter game ID</li
 ><li id="random">Enter random seed</li
 ><li id="save">Download save file</li
+><li id="load">Upload save file</li
 ></ul></li
 ><li>Type...<ul id="gametype"></ul></li
 ><li class="separator"></li