Add Appell functions?

Any interest or obvious route from the current implementations to adding [Appell function](https://dlmf.nist.gov/16.13) support to this package? I would mainly be interested in F1 for my application case.

It seems currently the only reasonable implementations of these functions are in the form of [this Fortran package](https://data.mendeley.com/datasets/9tvmt9d8yj/1), which also has [an R wrapper](https://www.rdocumentation.org/packages/appell/versions/0.0-4/topics/appellf1), and the as far as I can tell independent implementation in [the Python package mpmath](https://github.com/fredrik-johansson/mpmath) - see also the [documentation on mpmath](http://mpmath.org/doc/current/functions/hypergeometric.html).

Would be nice to be able to do this in Julia as well. Writing a wrapper would be straightforward but perhaps not in the spirit of the package?