global anchors, reflink

[tlienart]

closes #854 (cc @jkrumbiegel)

Syntax:

[this is a link](\reflink{some id}) 

on another page either

## some id

or 

\label{some id}

(white spaces are allowed).

Notes:

• if there's no header name clash or anchor name clash, this should just work
• if the same page has the same header multiple times, these get numbered and the number has to be referenced in the some id above
• if multiple pages have the same anchor, you get no guarantee of which link you'll get

Long story short, if you're using this, try not to have clashing references.