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

[Jacob Rus]

Michel Fortin wrote:

> That's why I think it'd be better to solve this while still allowing 

> nested brackets.

> 

> The other reason for supporting nested brackets is that they are already 

> supported for the content of the link, which is often the same as the 

> definition name. How unnatural would this be if you had to :

> 

>     In [his [distorted] view][], this isn't something to worry about.

> 

>     [his \[own\] view]: http://something.com/


What's wrong with just forcing users to write:

     In [his [distorted] view][hisview], this isn't something to worry
     about.

     [hisview]: http://something.com

I really don't get the advantage of having absurd reference names.

-Jacob