Use native messaging instead of websockets

[f1u77y]

I've migrated this application to using native messaging (reasoning is in #1). I've done a few thing that could be contraversial (pointed them in comments), so I'll change them if you think another way is better.

[brookst]

Ah great, sorry for not looking into this sooner.

This seems to work for me though I can't seem to get the stderr output via the Firefox console. I can see that the native app gets run once to send a ping back to the extension, then once again to service everything else. Is that the intended behaviour?

If you could run through clippy and rustfmt, that'd be appreciated. Otherwise this looks good to accept.

I'll see about setting up Appveyor for builds per #2.

[f1u77y]

I can see that the native app gets run once to send a ping back to the extension, then once again to service everything else. Is that the intended behaviour?

Yes, it is. But I've realized that "pong" answer in unnecessary check because checking if we can run native app with certain name, so I'll remove this behivaour eventually.