This commit fixes naive handling of relative links when proofing. The
problem here was the assumption that a relative link of foo.html that
was coming from a file at bar/fubar.md in the filesystem would always
appear at bar/foo.html.

That is not always true. If directory_urls was on, then it wasn't true.
Thus, manuzhang#46.

It was also not true when using relative links between blog posts when
using the mkdocs material blog plugin.

This commit changes the relative url lookup logic to be more robust and
in the process make the proofer work with both the mkdocs material blog
plugin and with directory_urls on.

It does this by storing additional information in the lookup files that
we use to find information about pages. We can now look up a file by its
src_uri entry, for example blog/posts/index.md and then use that to get
the dest_uri like blog/12/10/some-post/ and then use that dest_uri value
to robustly handle figuring out where a relative link is pointing.

Closes manuzhang#46
Closes manuzhang#70