Conceal not applied to math blocks in square bracket delimiters

[dzackgarza]

Hopefully, these screenshots capture what's happening:

Here. conceal seems to work for

• $x^2$
• ( x^2 )
• $$ x^2 $$

But not for [ x^2 ].

(Incidentally, vim seems to be picking up some sort error in the [ ] environment on the ^ symbol, perhaps this is related?)

Some possibly related commits:

• Three improvements to LaTeX highlighting #216
• Add support for tex_math_single_backslash #155