Support for th when converting html to Delta

[charliemidtlyng]

When converting html table to Delta the th tag is handled as text and therefore the table will be quirky.
It will hopefully be a small fix to support this (same places td is supported) since the rest of the table is converted as expected.

Steps for Reproduction

1. quill.clipboard.convert with a table that contains th element
2. Added table with th both as first column and first row - quilljs.com example code
3. Visit https://developer.mozilla.org/en-US/docs/Web/HTML/Element/table to see how it should work

Expected behavior:
One column per th

Actual behavior:
th tags end up as plain text

Platforms:
All browsers - this is not supported (i've checked the code)

Version:
2.0.3