Web demo should print banner, set line width

[julianhyde]

Following #30, web demo should print the banner and set line width. All available via properties:

Sys.show "showBanner";
> val it = SOME "true" : string option
Sys.show "banner";
> val it = SOME "morel-rust version X.Y.Z (rust version A.B.C, wasm32 core N.0)" : string option
Sys.show "lineWidth";
> val it = SOME "78" : string option
Sys.set ("lineWidth", 65);
> val it = () : unit

Line width 65 seems about right for the current demo page. Maybe the actual line width is in the HTML somewhere.