Modifiable processing element

[martijndwars]

Dynatable allows its user to specify a processingText which is then wrapped inside a <span>. However, span's do not allow block level elements, limiting the possible processing indicators. In particular, I wanted to use SpinKit, which was not possible.

This PR adds a processingTemplate with the <span> for backwards compatibility. By overwriting the processingTemplate, a user can get rid of the span and instead use e.g. SpinKit HTML.

[JangoSteve]

Is this selector change going to cause issues for calculating the total height of the processing indicator, if someone changes the template to be multiple sibling elements on the top level? E.g., if I call dynatable with processingTemplate: '<div>{text}</div><div>Please hold on</div>, this selector will only grab the first div. Maybe we want to change this line to be:

$span = $processing.children()

?