Merged changes from robpol86.github.io

I simplified "last updated" based on Michael Rose's suggestion. Instead
of normalizing dates and comparing differences, last updated will show
if page.modified is set. I didn't think that it was not required in the
front-matter to begin with. Now, if the user specifies it, the post will
show the last updated date above the content, even if it's the same
date.

Series was just a small change. If it's the same page being viewed,
instead of just the non-linked title, it will be a non-linked title in
<strong />. Without <strong /> it would probably confuse the user why
they cannot click on it before they realized it's the same page they're
looking at.
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Made author's avatar link to /about.
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Added Gaug.es support.
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Additionally, fixed small bug where `text` was different in paragraphs
and in lists. Added a line to main.css so `text` in lists looked the
same as if they were in <p />.
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Every page will now show the page's description under the site title
instead of the site's description. I think this is a better use of that
area instead of always showing the same thing.