Add playback speed selection

I've been checking in with this graph occasionally ever since it was mentioned on the MinuteEarth video. Since it's going through a larger range of data as time goes on, some people (namely, me) would prefer to go through it a little quicker.

Then I realised I also wanted to go through it slower in the latest last 1/4 or 1/8 of the data.

So I dropped in a speed selector that also defaults to the current speed. You can swap to faster or slower speeds. There is 'slow' and 'super slow', 'fast' and 'super fast'. It also supports URL construction/processing as with all the other parameters.

I'm okay if you don't merge the changes, but I feel others might enjoy the choice, hence my PR. If you want to merge the feature, feel free to alter as you see fit before merging.

aatishb / covidtrends

Add playback speed selection #201