feat: Supports capture area screenshot

[erickgskovik]

Fixes #389

I made selector and area optional arguments. In the implementation, I think that the pull requests shows the true intention of the screenshot region.

If full: true, It tries to capture full screenshot with full size browser window and then viewport if the full size is not accurate.
If full: false, :selector is preferred if defined. Then :area. Otherwise, the default will be the viewport.

Applying default scale (1.0) will not affect the result anyway.

[sandstrom]

@route friendly ping 😄

[route]

I'm back and going one by one from top to bottom ;)

[route]

Thanks, nice job!