Custom "read more..." links for teasers

To be more SEO-friendly it is useful to create more meaningful links
than "read more..."

Usage example:

Markdown:
  <!-- TEASER_END: Custom text -->

ReST:
  .. TEASER_END: Custom text

Default behaviour with TEASER_END should generate same links as
before.
docs for custom teasers
Merge branch 'master' of github.com:ralsina/nikola