Add support for covjson file upload.

[fsvieira]

Hi,

I am working with big covjson files, and copy+paste this files to the playground editor is a slow task, even my host text editor has problems handling this files.

My proposal is to add a file upload to playground and if its small it can show on playground editor but if its to big than it will not show at all.

[letmaik]

Adding a file upload button makes sense but not displaying the file content in the editor is out of scope for the playground I think, since the purpose of the playground is to learn and show CovJSON. Not showing the file defeats the purpose.