directive: `columnWidth` may be an element or selector.

The masonry [columnWidth][] option can be a:

> Number, Element, or Selector String

However, the current masonry directive pre-link function hardcodes a parseInt
coercion:

    columnWidth: parseInt(attrs.columnWidth, 10)

This fix still attempts to parseInt (so will still coerce string attribute values to
integers) whilst allowing non-numeric values (elements or selectors) to pass
through without error.

[columnWidth]: http://masonry.desandro.com/options.html#columnwidth