Stop capture

[RhinoW]

Checklist

• Code changes are only for the relevant bug fix or feature
• New code lints (via npm run lint) without any errors or warnings
  (every line errors "Expected linebreaks to be 'LF' but found 'CRLF'" which I guess is because I am on windows? No other errors)
• The corresponding issue is Can I use timesnap without the duration parameter? #32 Can I use timesnap without the duration parameter? #32

Description

In my use case (and Issue #32) I don't know exactly how long the capture should be before hand.
This PR allows page to use window.stopCapture() to control when capture ends.

[tungs]

Thanks for filing this. I like the idea of being able to stop capturing from the captured web page. There seem to be some minor issues (like typos), but my biggest concern is polluting the global namespace, so I'd prefer to use something more specific and less likely to conflict than window.stopCapture. I'll think about it this week.

[tungs]

I'm still thinking about what I'd prefer the name to be instead of window.stopCapture(), but here are the other changes I have in mind. I'll do another review when I decide on a different name.

[RhinoW]

This may be a better way to handle - #24 - a captureWhileSelectorExists option.

[RhinoW]

Decision between

• Add something like window.startTimesnap() window.stopTimesnap() methods - easy but pollutes global namespace
• Ashboys captureWhileSelectorExists idea - doesn't pollute but slightly more faff from rendered page (have to create/delete some element rather than calling direct).

[tungs]

After mulling over the idea of polluting the global namespace with arbitrary names, I think a somewhat cleaner implementation is to allow the user to specify the name from the command line or config. Compared to the alternatives, this still allows for the stop functionality to be called from a command line invocation, while polluting with a user-chosen name.

[tungs]

Instead of using the set name stopCapture, I think it's cleaner to allow the user to specify the name of the stop function via config.stopFunctionName from node or --stop-function-name from the command line.

The command line and node API sections of README.md need this addition, as does cli.js, though I can make these changes.

Let me know what you think.