[FEATURE] Set align attribute for <th> elements

[JakeQZ]

Since this is set for <td> elements, it makes sense to also set it for <th>
elements, as both represent table cells. Although these attributes are
obsolete, there may still be some legacy email clients (e.g. older versions of
Outlook) that need it.

[JakeQZ]

w3resource suggests this attribute can be set on any of the table content elements (though seems to omit <caption> from its list). However, for <col> and <colgroup> it is not analogous to a CSS property, because the elements are not ancestors of the table cell elements. I'm unsure whether it is worthwhile applying the attribute to <tbody>, <thead>, <tfoot> and <tr> for it to be 'inherited'. I'm also unsure about applying it to <caption> as that was omitted from the w3resource list. But at a minimum it seems logical that it should be applied to <th> as well as <td>.

[JakeQZ]

@oliverklee, #1004 reminded me this was one of the 'fixes' I'd promised a PR for ages ago (though I think it's an enhancement rather than a fix). Hopefully there won't be any merge conflicts with that PR.