pelican-bootstrap3: extended sidebar images to include images with attributes.

[rene00]

Add images with attributes to the sidebar. In pelicanconf.py add a SIDEBAR_IMAGES_WITH_ATTRIBUTES dictionary:

SIDEBAR_IMAGES_WITH_ATTRIBUTES = {
    'image-with-link': {
        'href': 'http://example.org',
        'src': 'images/example.jpg',
        'width': '90%',
        'class': 'img-rounded'
    }
}

Which should render:

<a href="http://example.org"><img width="90%" class="img-rounded" src="images/example.jpg"/></a>

[almet]

Great, thanks for taking the time to submit this (and sorry for the delay on reviewing the changes :-))

Can you consider updating the README in order to have users of this theme have the information that they can use this new feature? Thanks!