Add Dateish plugin

This plugin allows arbitrary metadata fields to be automatically
converted to datetime objects, just like the 'date' field.
changed git  submodule config to use https instead of git

changed git  submodule config to use https instead of git
git on submodule result in erros

see: https://travis-ci.org/pythonclub/pythonclub.github.io
fix tabbing
Merge pull request #275 from luzfcb/patch-1

Change git submodules to use https instead of git
added pelican-langcategory plugin
Merge pull request #276 from fly/fixtabs

fix tabbing
Add language highlighting option to notebook tag

Update documentation and fix small errors
Add tests for notebook tag regex

Update SYNTAX string to match new regex

Explicitly outline necessary order for tag options

Addresses concern in
https://github.com/getpelican/pelican-plugins/pull/283#issuecomment-53731077
Merge pull request #283 from isms/liquid-notebook-language

[liquid_tags:notebook] Add language highlighting option
Merge pull request #281 from CNBorn/langcategory

Add pelican-langcategory plugin as a submodule
Update timestamps of generated feeds as well
Merge pull request #286 from arty-name/touch-feeds

Update timestamps of generated feeds as well
Update Readme.md
Fix sitemap with disabled pages (fix #206)

Recommendation in pelican < 3.4 was to disable pages using
a False boolean (http://docs.getpelican.com/en/3.3.0/settings.html#basic-settings)
It's now recommended to use '' and the sitemap plugins tries to
join paths using those variables, which won't work with a bool.
This checks that we have a truthy value before joining.
Merge pull request #290 from Keats/master

Fix sitemap with disabled pages (fix #206)
Merge pull request #288 from wilsonfreitas/master

Improve rmd_reader plugin README
Handle subdirectories in thumbnailer.
Change MathJax URL from deprecated Rackspace CDN

Use new cdn.mathjax.org URL

More information available here:
http://www.mathjax.org/changes-to-the-mathjax-cdn/
Merge pull request #292 from isms/liquid-notebook-language

Change MathJax URL from deprecated Rackspace CDN
Update bootstrap_rst_directives.py

changed alert-default to alert-warning in docs, there is no 'alert-default' in bootstrap.
ADD: sort_tags plugin

This plugin adds `tags_sorted_article_length` to the context,
which is a list of tupels (Tag, [Articles]) that is sorted 
by number of Articles first and Tag second.