Add webactions support

[barnabywalters]

Wrapping the subtome button code in an <action do="subscribe"> element would allow people using a web actions implementation to subscribe using their chosen application, falling back to subtome for everyone else.

Whilst I’d love to see this implemented, I’m aware web actions and the <action> element is highly experimental and so I was uncertain about adding it to the suggested copy/paste snippet — thoughts about how to present this to publishers would be appreciated (are my concerns legit? should we just throw it out there and see what happens?)

See also:

• http://indiewebcamp.com/webactions
• https://github.com/barnabywalters/web-action-hero-toolbelt

[singpolyma]

I'd be very concerned about including invalid markup by default