Feature/Issue 90: Add support for handling browser forward/back buttons

[chiaminchuang]

Issue 90

Description of changes:

• Add popstate event listener to address forward/backward event. Invoke s3list() when window.location changes
• Modify folder click event callback.
  • Instead of invoking s3list() directly, change window.location -> trigger popstate event -> invoke s3list()

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[simsong]

Hm. Wish I had seen this when I implemented #131 .
Is this project abandoned by its maintainers?