Allows for custom headers to be sent, helpful for Authorization

[bennett000]

[bennett000]

I am not certain why these errors are occurring. I was certainly responsible for some linter errors that I fixed but the other errors seem to be entirely around node code...

[sindresorhus]

Needs a test and be documented.

[bennett000]

I can document this no problem. I have two challenges with testing:

• The existing tests pass locally but CI is blowing up
• I'm not sure how to go about testing the header code since the current test suite is setup for integration testing and ends up passing file:/// urls directly to phantom.

Ultimately I don't have strong opinions on this since we ended up going with a third part a11y testing service that uses humans.

[seanmwalker]

My apologies, I didn't see this PR. I did submit one last night that I think gives the same capabilities. That one allows for a phantomjs configuration file to be specified. Using that you can set headers etc... Let me know if that helps at all or if it's not the same issue. Thanks