Tightening the rules for literal `[` and `]` chars in link ids

[Jacob Rus]

John Gruber wrote:

> *   We could also just tighten the rules for link ref IDs all the

>     way, and outright ban literal `[` and `]` chars in the IDs.


It has always seemed to me that links should just be 
"[A-Za-z][A-Za-z0-9._-]+", and maybe 2-3 other characters, but as few as 
possible.  There's no reason for a reference name that no one will ever 
see to have a creative title with lots of punctuation.

-Jacob