added thumbnails plugin
fixed function description
thumbnails plugin: crop image if both width and height defined
thumbnails plugin: added 3 crop types
thumbnails plugin: make resources under content directory instead of generating
them in deploy directly, make all thumnails in single place(content/.thumbnails)
for simple maintenance but keep original deploy path to preserve naming logic in
generated site
Merge pull request #81 from gfuchedzhy/thumbnails

Thumbnails
Allow docutils configuration overrides

Any settings specified in the site configuration under
``restructuredtext`` are passed to ``publish_parts``, and can
be used to customise html output. For example::

    # site.yaml
    restructuredtext:
         highlight_source: True
         initial_header_level: 2
         doctitle_xform: False

Note that ``hightlight_source`` is not a docutils option and is
handled independently.
Merge branch 'rst-config'