Receives a computed CSS property value
for the given element, casted as a number.
The value is casted by removing the property's unit (which is usually px for computed properties).
The result is then parsed as a floating point number.
Returns undefined if the property value is missing, or if it cannot
be parsed as a number.
Examples
When requesting a single CSS property, its value will be returned as a string:
value = up.element.style(element, 'font-size')
// value is '16px'
value = up.element.styleNumber(element, 'font-size')
// value is 16
