Added functionality that allows you to programmatically create a screenshot

[dehli]

For this function, I made a few changes.

1. First, I made sure all of the CSS styling for the SVG elements were applied inline. This was so that the SVG elements could be merged into a canvas retaining their styling.
2. Then I created a mergeParcoords function which merges the canvases and the SVG elements together into one canvas.
3. The merged canvas element can be used by the consumer in whatever way they want. I added a download example which shows how it can be used to download a screenshot of the chart.
4. I added an example to showcase the functionality.

Let me know your thoughts. Thanks!

Notes:

• I had tried using a style sheet inside of the root SVG tag, however I encountered issues with this when using IE11.
• Also, I couldn't figure out how to get saving working with IE (I get SecurityError whenever I try to save the contents of the merged canvas). This is the main reason why I chose to have the function return a canvas instead of just saving the screenshot.
• We could later use this to create high quality screenshots (Issue #310)

[syntagmatic]

It looks like the canvas for the screenshot needs to pull in the scale factor for retina displays. On a rMPB the SVG looks good, but the canvas ends up at a different size. https://github.com/syntagmatic/parallel-coordinates/pull/288

[dehli]

Thanks @syntagmatic! I forgot to account for retina displays. I'll make a fix for it.

[dehli]

Hi @syntagmatic, I believe I fixed the issue. I don't have a retina display to test it but I tested it by using the zoom feature in the browser. Let me know what you think!

[dehli]

Hey @syntagmatic, would it be okay with you if I went ahead and merged this PR?

[syntagmatic]

Sorry I was on vacation! Let me do a quick test this evening on my macbook, just to check.

[dehli]

No worries! @syntagmatic I wasn't sure if you had forgotten about this. Hope you had a good vacation!

[syntagmatic]

Works for me! Nice job!