Add Internal Links

[killercup]

It should be possible to link to other files in the documentation using relative URLs.

E.g., the comment in

###
# Set the answer to the [correct value](../magic_numbers.coffee#answers).
###
answer = 42

should render like

Set the answer to the <a href="../magic_numbers.coffee.html#answers">correct value</a>.

Determining whether a link points to an existing file (and anchor) is secondary.