Add support for amsmath cases enviroment

**How should the input text representation look like**
> Example: f(x) = \\begin{cases} 1 \\; if \; x < 2  \\\  0 \\; if  \\; x \\le 2 \\end{cases}

**How should the ASCII art output look like**
```
     ⎧ 1 if x < 2
f(x)=⎨
     ⎩ 0 if x ≥ 2
```

useful for functions, such as characteristic functions
