Add a sniff for pretty URLs

[fulldecent]

If built page includes a match:

m/href="[^"]+.html"/

which doesn't also match:

m@://@

then that is an error.

---

Or in normal English: you must not link to relative HTML files with the .html extension.

[fulldecent]

This could be implemented with #122

[fulldecent]

Here is how to do the search today with Perll

perl -0777 -ne 'while (/href="[^:"]+html?"/sg) { print "$ARGV: $&\n"; }' **/build/*.html

(our public-websites does not have any violations today)

[fulldecent]

This is at

https://github.com/fulldecent/github-pages-template/blob/9ccd32ebefada445e1cccc85b9cfb01c9d0e9812/test/dirty-words.json#L15