Supporting full-page screenshots

# Story

As a Developer
I want to capture full-page screenshots (using native browser functions)

# Acceptance Criteria

Given latest ChromeDriver or GeckoDriver
When I pass `full_page: true`
Then a full page screenshot is captured (as opposed to a normal screenshot that captures the visible viewport only)

## Example:

`take_screenshot(name: "screenshot-name", full_page: true)`