Implement \qPochhammer{a}{q}{n}

[poortho]

From @HowardCohl on April 1, 2016 19:58

For instance,

(a;q)_n to be replaced with \qPochhammer{a}{q}{n}

It has to look for a semicolon to separate the a and the q.

Copied from original issue: DRMF/texvcjs#39

[poortho]

finished with #43