Enhanced text mode for the new HTML canvas terminal

[Ethan A M. <merritt@u.washington.edu>]

I have added enhanced text support to the HTML canvas terminal
and created an alternative font file
	canvasmath.js
that extends the basic ascii character set in canvastext.js by 
adding Greek letters and math symbols.  
See, for example

  http://skuld.bmsc.washington.edu/~merritt/gnuplot/canvas_demos/approximate.html
  http://skuld.bmsc.washington.edu/~merritt/gnuplot/canvas_demos/enhanced_utf8.html

All characters are accessible using UTF-8 encoding.
I had to choose some set of characters to include, since including
absolutely everything would make for a very large file.
The current set is here:

  http://skuld.bmsc.washington.edu/~merritt/gnuplot/canvas_demos/utf8.html

If you notice a missing character that should be included, please speak up.
It's just a matter of hunting it down in the poorly-indexed Hershey font set.

-- 
Ethan A Merritt