Add Dateish plugin

This plugin allows arbitrary metadata fields to be automatically
converted to datetime objects, just like the 'date' field.
Add GZIP_CACHE_OVERWRITE option to overwrite original files

This option is useful when hosting static content on a service such as
Amazon S3. Setting to True will overwrite the original file with
the gzip-compressed file. In order to correctly serve the file, the
Content Encoding must be set to gzip by the hosting service.
Fixed #300. Sitemap plug-in: '%s' should be '%z' for printing out timezone.
Merge pull request #309 from barrysteyn/render_math

Render math update
Merge pull request #306 from andyli/fix-sitemap-timezone

Sitemap plug-in: '%s' should be '%z' for printing out timezone
Merge pull request #298 from jkrshw/feature/gzip-overwrite

Add GZIP_CACHE_OVERWRITE option to overwrite original files
document required external libraries for PILLOW
format inline code blocks
Merge pull request #259 from mcgid/dateish-plugin

Add Dateish plugin
Merge pull request #284 from fly/thumb_doc

Thumbnailer doc enhancement
added pandoc_reader (pandoc markdown reader plugin) as submodule
Merge pull request #278 from liob/pandoc_reader

Add pandoc_reader plugin as a submodule
FIX: Sitemap offset-naive datetimes(modified) error
Added files for new Video Privacy Enhancer Pelican plugin.
Added files for new Codeblock Line Numbering Pelican plugin.
Merge pull request #311 from zhangliyong/master

FIX: Sitemap offset-naive datetimes(modified) error
Merge pull request #216 from publicus/youtube_privacy_enhancer_plugin_development

Added files for new Youtube Privacy Enhancer Pelican plugin.
Merge pull request #217 from publicus/better_codeblock_line_numbering_development

Add files for new Codeblock Line Numbering Pelican plugin.
Render_math now working with markdown versions >= 2.5