RST reader: Remove duplicate 'http' in PEP links

The generated link to PEPs had a duplicate 'http://' in its URL.
Org reader: Recognize plain and angle links

This adds support for plain links (like http://zeitlens.com) and angle
links (like <http://moltkeplatz.de>).
Org reader: Read anchors as empty spans

Anchors (like <<this>>) are parsed as empty spans.
Org reader: Enable internal links

Internal links in Org are possible by using an anchor-name as the target
of a link:

[[some-anchor][This]] is an internal link.

It links <<some-anchor>> here.
LaTeX writer: Mark span contents with label if span has an ID

Prepend `\label{span-id}` to span contents iff `span-id` is defined.
Merge pull request #1265 from tarleb/org-links

Improvements handling of internal links