Fixes affix plugin to allow a function for offset.

Created by: joshma

The current affix plugin allows for a function in the top or bottom options:

$el.affix({
  offset: {
    top: function() {/* Some function */},
    bottom: function() {/* Some function */}
  }
});

but not as a single parameter for both top and bottom:

$el.affix({
  offset: function() {/* Some function */}
});

This PR evaluates offset() first if it's a function (and updates the docs to match).