Support sigiled links

[darioteixeira]

It would be useful if core Lambdoc and all its readers would support special links formed by a sigil such as @ or # preceding a word. Consider for instance Github, which transforms a user name sigiled by @ into a link into the user's page.

A sigiled identifier would be represented as a new Sigiled variant in Lambdoc_core.Inline, and the mapping between sigil and actual link would be performed by the extension mechanism. Moreover, to avoid forcing users to escape every control character, only rarely used characters such as @ and # may be used as sigils. In fact, I would suggest that only those two characters are actually used.