Add provider field in json output

[ilyaglow]

Hey there,

I had a need to know from which provider which URLs came from, so I added provider field in json output.
Unfortunately, I haven't found a way to do it in a backward compatible manner.

Also, provider names now referenced as variables instead of just plain strings so it's less likely to shoot yourself in the foot in the future.
I hope this PR is going to be helpful.

Cheers!