Support filling forms via right-click context menu

[maximbaz]

There are cases when Browserpass doesn't recognize username, password or entire forms.

Let's consider if it would be possible to allow user to right click in a field and e.g. select "Browserpass > Fill password".

Originally suggested here: #62 (comment)

[Gigadoc2]

This could also be helpful for pop-up windows, where the browser does not show the extension icon.

[oktayacikalin]

Any plans for this?

[fkneist]

I did a proof of concept, feel free to have a look #282