Use text-align over deprecated align attribute for td and th

[popekim]

align attribute is deprecated, and the text aligning of and elements should be done by text-align

[gjtorikian]

I'll let @kivikakk have final say here. The goal of this library is:

to model the upstream cmark-gfm as closely as possible in terms of code structure.

cmark-gfm toggles this alignment with an option, so that may also be needed here.

(Personally, damn the past and accept the future, LGTM)

[kivikakk]

Yes, I'd be happy to accept an option to toggle this; I don't particularly want to break executable spec compat (see CI) or change output for all existing users.

[kivikakk]

Closing since there's no option forthcoming! Thanks for your efforts here.