Added feature to upload image by URL

[yugalkaushik]

Fixes #387

Changes: This PR adds feature as suggested in Issue #387 by adding the ability to upload files by URL in the p5.js Web Editor.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

[yugalkaushik]

@raclim can you please check this PR

[raclim]

Thanks for checking in @yugalkaushik! Will try to take a look before the end of this week!