Pretty printing of elliptic integrals

Pretty printing of elliptic integrals

```
In [3]: elliptic_pi(3, 4/x,6)
Out[3]: elliptic_pi(3, 4/x, 6)

In [4]: pprint(_, use_unicode=True)
 ⎛   4│ ⎞
Π⎜3, ─│6⎟
 ⎝   x│ ⎠

In [5]: pprint(_, use_unicode=False)
  /   4| \
Pi|3, -|6|
  \   x| /
```