Allow setting custom headers

[colearendt]

Related to #77 , it would be nice to be able to set arbitrary headers (i.e. not just userAgent). Unfortunately, per

• http://docs.casperjs.org/en/latest/modules/casper.html
• https://holsson.wordpress.com/2016/02/04/adding-a-custom-http-header-using-casperjs/
• https://stackoverflow.com/questions/44218995/casperjs-phantomjs-modifying-headers
• https://stackoverflow.com/questions/10114925/phantomjs-doesnt-send-authentication-header
• http://darrendev.blogspot.com/2013/04/phantomjs-post-auth-and-timeouts.html

It seems that pageSettings does not work for setting arbitrary headers (at least not with the incantations I tried), and passing the headers to .open() does not send the headers with follow-on requests. As a result, the preferred (and workable) approach was to add the following to webshot.js before casper.start():

casper.on('started', function () {
  this.page.customHeaders = {
    "Header" : "Value" 
  };
});

This is also anecdotally related to #49 , because I am using headers for authentication to RStudio Connect (in lieu of clicking through auth and then saving a cookie).