Added code to retrieve horizontal letters.

This is new code. The new function horizletters() looks an
awful lot like vertletters(). I did confirm with the 'Check'
console.log() that this working properly.
Added code to retrieved diagnol letters.

I also fixed a nasty bug in which the diagnol for loops
(for filling the highlight, and retrieving the letters) was using
"..." instead of "..". This prevented the last letter in the diagnol
from getting filled.
Added code to add horizontal words.

The code smells funny to me. It really feels repetitive. I'm not going
to refactor everything just yet. Diagnols are still a bizarre case, and
having things separate (horizontal, vertical, diagnol) still feels
right.
Implemented adding diagnol words.

Not all diagnols, however. Just 'NE' and 'NW'. Added test words 'snow'
and 'leaf'. As I type this, the program is nearly finished. What I
really have to come to grips with is making a "make puzzle" page.
Added URL to README.md.

How could I have forgotten this?
Added footer pointing to the source code.

I also updated the instructions. You can drag the words now!