feed: Remove <table> elements from HTML mail

These were not semantically correct ;).  Based on a patch by Rui Carmo
[1].

[1]: https://github.com/rcarmo/rss2email/commit/2a015bce9d701035b9af874bd56c46f92382e668

Based-on-patch-by: Rui Carmo <rui.carmo@gmail.com>
Signed-off-by: W. Trevor King <wking@tremily.us>