handle nested braces in alignment specification

[Alex-Jordan]

This fixes a bug with niceTables. Without this commit, if you specify an alignment string like *{3}{p{2in}} somewhere in a niceTables table, it will break. The regex treats this: {p{2in} as a matched brace pair with p{2in as the contents.

This commit changes the regex to balance the braces.

Try this example:

DOCUMENT();
loadMacros(qw(PGstandard.pl PGML.pl));
BEGIN_PGML
[# [. Cell .]  [. Cell .]  [. Cell .] #]*{align => '*{3}{p{2in}}'}
END_PGML
ENDDOCUMENT();

[drgrice1]

Impulsive approval!