Fix invalid atom feed id:tag

Fixes #1039: The atom feed generated produces an invalid attribute
id:tag when SITEURL has something different that http or it contain
a subdirectory, such as:
  * https://myblog.com/
  * https://mysite.com/blog
  * http://mysite.com/blog
Add 'asciidoc' to the ASCIIDoc possible extensions
Merge pull request #1140 from gpoo/fix-invalid-atom-feed-uid

Fix #1039 Atom feed for sites using HTTPS or a subdirectory
Build Pelican wheels in universal format

Including a binary wheel when uploading Pelican to PyPI will speed up
installation for the Pelican community. This change ensures that the
Pelican wheel is compatible with all supported Python versions. For more
information, see: https://pypi.python.org/pypi/wheel
The modified attribute no longer has a default value.
This allows for templates using {% if article.modified %}
instead of {% if article.modified != article.published %} .
For creating a CNAME, directory name should be extra

The name of the directory do not match subsequent commands. hence renaming extras to extra. A one character change
Merge pull request #1163 from salilpa/patch-1

For creating a CNAME, directory name should be extra
Merge pull request #1147 from woshilapin/master

Add 'asciidoc' to the ASCIIDoc possible extensions
Add 'adoc' as a file extension for asciidoc

This is to match the behaviour that GitHub have
Merge pull request #1161 from florianjacob/modified_no_default

The modified attribute no longer has a default value.