Matrix environment support

[ForNeVeR]

This is a successor task for #100.

In #155, we've successfuly implemented commands such as \matrix, \cases and \pmatrix.

Also, there's an internal support for environments. Let's make it official and add all the matrix environments from that article:

• [ ] \begin{matrix},
• [x] \begin{pmatrix},
• [ ] \begin{vmatrix},
• [ ] \begin{Vmatrix}

should all work.