markdown: allow cell alignment in tables

Mistletoe supports Github's tables extension, including cell alignment
(see [1]). However, the sanitizer currently does not allow the align
attributes on the table cells, so the alignment never renders.

Add the align attribute to the list of allowed attributes for td and th
element, so that cell alignment works as expected.

[1] https://github.github.com/gfm/#example-199

srht/markdown.py

@@ -180,6 +180,8 @@ _sanitizer_attrs = {
     "h4": ["id"],
     "h5": ["id"],
     "h6": ["id"],
+    "th": ["align"],
+    "td": ["align"],
     "img": _img_filter,
     "input": _input_filter,
     "div": _div_filter,