Remove `background-color` when `bgcolor` present

While `background-color` has is quite reliable, HTML `bgcolor`
attribute has the best support across the board.
(https://litmus.com/blog/background-colors-html-email)

This PR attempts to slightly reduce the template’s entropy by
consolidating background colors into a single declaration.
Merge pull request #2 from TedGoas/master

Remove `background-color` when `bgcolor` present