Ensure isBlank checks for significant elements within the given node

[domchristie]

isBlank currently checks:

• if the element is meaningful i.e. a 'A', 'TH', 'TD', 'IFRAME', 'SCRIPT', 'AUDIO', 'VIDEO',
• if the textContent is blank,
• if the element is a void, and
• if the element contains a void element

This works for most cases, although fails when the element contains a meaningful element and has a blank textContent e.g.

<figure><iframe src="…"></iframe></figure>

(Adapted from: #293)

This pull request checks if the element contains a meaningful element. It also adds 'TABLE', 'THEAD', 'TBODY', 'TFOOT' to the list of meaningful elements.

Fixes #293
Fixes #276
Closes #277
Closes #311

[jerelmiller]

First of all, thanks so much for this library! I've found it suppppper useful.

By chance, do you know when this fix might get published? I could really use this. Thanks!