Add "TextColor" or similar option for button

[Alexey Kamenskiy]

New Gmail web client seems to differ from before in how it handles button shown in email.

In previous version the button text was shown colored in accordance with theme, but now it shows as:

From what it looks new Gmail web client adds its own style that overrides the embedded CSS from hermes theme:

This is fixable by adding inline CSS to the button color: white; (for example) to make it look as below:

Or I think this could be fixed by doing what #3 suggests and inlining all CSS.

[Alexey Kamenskiy]

Closing since merged.

Thanks for such a nice package!