Official support for markdown and plain text descriptions

[takluyver]

Fixes #166.

Until recently, PyPI only supported rst descriptions, falling back to plain text if the rst was invalid. Now a new metadata field Description-Content-Type lets us specify whether the description is rst, markdown or plain text. This new code uses that, picking the content type based on the file extension (.rst, .md or .txt). If the extension is something else, Flit will warn the user and leave the field unset, so it's up to PyPI what to do with it.

Markdown support has been a secret feature of Flit for some time, by using pandoc to convert markdown to rst (thanks @Carreau!). This makes it an official feature, and gets rid of the code to call pandoc, since PyPI can now handle markdown natively.

[Carreau]

There is now a variant type being GFM (default. GitHub FLavored Markdown), or CommonMark.

I don't think it will import (much), but we should have an option to set the variant.

I' havent dive in the code, runnign to lunch.

[takluyver]

This is partly laziness, but I'm inclined to ship it without that option (i.e. all markdown will be GFM for now), and add it later if people want it.

[Carreau]

The charset seem to also be an option I'm in favor of enforcing utf-8 and/or adding a markdown-variant flag in a subsequent PR.

[Carreau]

+1 to merge as is, we can take care of the md variant in a separate PR if you like.

[takluyver]

The spec says about charset:

The only legal value is UTF-8. If omitted, it is assumed to be UTF-8.

We already read the file as UTF-8, so it will fail if you point it to a file that isn't valid UTF-8. So I don't think there's anything to do on that front.