Add an option to make Stamp & Send the default sending option

[jopasserat]

Ability to take over the plain Send when keystrokes like Ctrl + Return are hit for example.

[jonasbits]

Ctrl + Return ( Cmd + Return on macOS ) and starting to generate hashcash stamp on first valid email address in TO/CC fields would really help to ramp up usage.

I would also suggest generating a dataurl QR-code to make it more clear that the email has a stamp.

https://bz.apache.org/SpamAssassin/show_bug.cgi?id=4674
This is bugzilla 4674 from SpamAssassin that states that hashcash parsing is not enabled by default. Lets try to change that.