wip: svg now saves static svg for plotlyjs + reliable IJulia output

[piever]

Solves #750. I've added a specialized method to display html plots for PlotlyJS, so that it doesn't default to svg. Now svg can save the static image. The html can still be saved using for example savefig("test.html").
As an added bonus, this method of showing html on IJulia doesn't have the flickering problem I used to experience when combining it with Interact.

[piever]

EDIT = Actually, it seems to recreate the problem of plots disappearing when refreshing the page. It's probably better to not merge yet and I'll try to see if I can find a solution that works more reliably.

[mkborregaard]

This is a recurrent issue with plotlyjs (cf #725) - it may not be caused by this PR. But, if you think you can unwind that issue with the changes you're doing here that would be really great!

[piever]

At least for me it started not working reliably with #604 (and before that #591 ), which were the PR to fix the issue that plots are gone after refresh. By removing it I think it restarted working better and flickering with interact is gone but I got the refresh issue again. Not really sure what's the way to go.
This and this are the relevant issues. The solution for PlotlyJS was this commit I believe with the idea of exposing a function that can call the javascript at all time to force it to be reloaded. I'm not sure how that helps, I guess @pabloferz or @sglyon (who fixed those two issues) understand this stuff much better than I do.

[mkborregaard]

To all open PRs: Due to the change to 0.6 it appears that all PRs will have to be rebased onto master for Travis tests to run.

[piever]

I'll close this one and split this PR in two parts (one which takes care of being able to save as svg, the other one will try to remove the existing "hack" to display on Jupyter notebook)