TOC API is overcomplicated

[techtonik]

Instead of

import mistune
from mistune_contrib.toc import TocMixin
class TocRenderer(TocMixin, mistune.Renderer):
    pass
toc = TocRenderer()
markdown = mistune.Markdown(renderer=toc)

toc = toc.reset_toc()

import mistune
from mistune_contrib.toc import TocRenderer
markdown = mistune.Markdown(renderer=TocRenderer)

[techtonik]

Take a look how nice interface was turned into somewhat messy https://github.com/gratipay/inside.gratipay.com/pull/339/files

[lepture]

@techtonik I used TocMixin instead TocRenderer because we will always use a custom renderer.