Allow creating and adding `pygments.styles`

[fatteneder]

Instructions on how to create styles in python are given here: https://pygments.org/docs/styledevelopment/#creating-own-styles

Loading styles into pygments requires tinkering with setuptools, see https://pygments.org/docs/plugins/

I managed to get it to work in python. It remains to figure out how to load it into PyCall, since the new styles have to be installed as a plugin using setuptools.

Open questions:

• Updating a style file requires to rerun the installation of the plugin. Can we do this from within the same julia session using PyCall or do we have to restart the REPL everytime we change the style?
• Can we provide an environment variable and/or default directory in which setuptools can look for style files?

[fatteneder]

There is a discussion going on about alternative ways to register custom lexers or styles in pygments: https://github.com/pygments/pygments/issues/1096