support for custom User-Agent string

[kylewm]

I'd like to send my own user-agent string instead of the default python-requests one. (Wordpress installations in particular tend to block requests's UA).

[sknebel]

Piggy-backing on this, an option to pass custom headers that are used for all requests would useful and enough to solve this.

In requests, the user-agent is changed by explicitly adding an User-agent header, and it could be used for other purposes, like passing X-Forwarded-For.

[bear]

both are great suggestions - I'll add them quickly

thanks!