[suggestion] Support HTTP Headers

[pglezen]

It would help if I could add an HTTP header value to the gron command so that I could specify an API key. As it is, I use curl and pipe its output into gron; that works.

I'm thinking of being able to run

gron -H "x-api-key: some-secret-api-key" https://my.protected.service.com

[adamritter]

I added this feature to https://github.com/adamritter/fastgron