Add a template variable to specify custom CSS file.

This patch is inspired by CUSTOM_CSS variable in
pelican-bootstrap3. The variable allows the user to specify a custom
CSS file, for styles used within the articles and pages.
Reorder rel and href attributes in link tag.

Reorder rel and href attributes in link tag, to match the surrounding
code.
Merge pull request #2 from bravegnu/site-license

Add a template variable to specify custom CSS file.