[feature] Wait for javascript to take screenshot

[facultymatt]

The proposed feature allows the option to wait for a javascript function to be called before taking a screenshot.

This would work like the takeShotOnCallback method from node webshot, a similar project that uses phantomJS exclusively, see https://github.com/brenden/node-webshot#options.

[nikmmy]

Hi Vlasdislav,

Thank you for creating Manet! I use PhantomJS extensively and this is a nice addition to it. I was wondering if you are working on this enhancement or is it open for being taken up. I would love to work on it, but I do not want to end up duplicating the fix, in case you plan to release one.

[vbauer]

I'm opened for PR, if you'd like to help this project. :)

[vbauer]

JFYI: It could be useful #55