Newer versions of Jekyll introduced the site.baseurl configuration
option [1], which is useful for testing the site locally. This commit adds
support for site.baseurl to this theme.

site.url is now used for the protocol and domain name only, without a
trailing slash. site.baseurl, if specified, has a leading slash and no
trailing slash.

In many instances, site.url has been replaced by site.baseurl where the
protocol and domain are not necessary (e.g., <a>, <img>, etc.). site.url
is now used in the following places:

- the canonical link
- the feed link and within the feed
- sharing, including Open Graph
- search

Some advice for where to use each comes from this StackOverflow post
[2], and other changes were based on the scaffolding generated by the
latest version of Jekyll (3.0.3).

[1]: https://jekyllrb.com/docs/upgrading/0-to-2/#baseurl
[2]: http://stackoverflow.com/a/27400343