Fixes affix plugin to allow a function for offset.

The current affix plugin allows for a function in the top or bottom options:
```javascript
$el.affix({
  offset: {
    top: function() {/* Some function */},
    bottom: function() {/* Some function */}
  }
});
```

but not as a single parameter for both top __and__ bottom:
```javascript
$el.affix({
  offset: function() {/* Some function */}
});
```

This PR evaluates `offset()` first if it's a function (and updates the docs to match).