Support for \begin and \end named environments

Would be great if the engine/parser could support \begin and \end **named** environments.

Examples:

\begin{pmatrix} a_{11} & a_{12} \\ a_{21} & a_{22} \end{pmatrix}

\begin{array}{c|c} 1 & 2 \\ \hline 3 & 4 \end{array}

f(n) = \begin{cases} n/2 & \quad \text{if } n \text{ is even}\\ -(n+1)/2 & \quad \text{if } n \text{ is odd} \end{cases}

\left(\begin{ array}{c} n \\ r \end{array} \right) = \frac{n!}{r!(n-r)!}

## TODO[F]
- [x] `pmatrix` (see #329)
- [ ] `array`
- [ ] `cases`