Config: Add option for setting extra HTTP Headers

[pmeenan]

It is useful to provide custom HTTP headers for configuring server settings (some accelerator providers use it to turn features on or off), to provide BASIC Auth headers or in some cases to provide session cookies to bypass a login.

It would be helpful to be able to pass a config of headers to override and if present pass it through to Network.setExtraHTTPHeaders when the emulation configs are done.

[patrickhulce]

related #1418

[kaycebasques]

Somebody requested this on SO: https://stackoverflow.com/questions/46584644/sending-headers-with-lighthouse

[paulirish]

now fixed via #3732

Check out the docs for --extra-headers in the readme or lighthouse --help