Support for URL extensions

[morisy]

DocumentCloud currently supports a number of special extensions that can just be added to the DocumentCloud URL (some are documented here):

.txt: Shows just the OCRd text as a txt file.
.pdf: Links to the raw PDF
/p2: links to specific page
/p57/a42283: links to a specific note

And I think there's also something that shows you just the text from a specific page but can't find it?

[freedmand]

Goes hand-in-hand with embeds. Good to note the .txt and .pdf behavior, which can be a redirect

[eyeseast]

Closed via #1005