Expose headers option to CLI

[frvge]

Please consider exposing adding request headers to the CLI.

I tried to get it working, but I'm hitting a wall:

.option(
	'-H, --headers <headers>,
	'a request header to pass in to the original URL. Add multiple occurrences for multiple headers. Split value by colon',
	processHeaders,
	{} // memory
)

... snip...

function processHeaders(headers, memo) {
	let [key, val] = headers.split(':');
	memo[key] = val;
	return memo;
}

to be used like node bin/pa11y.js -H "x-test:1" -H "x-test2:2" http://www.myurl.com

Now it's still possible to do it via the config file, but that is cumbersome if there are runtime variables involved.