Add "Re-take snapshot" button

This addresses: https://github.com/trotzig/diffux/issues/55

Sometimes you may be looking at a snapshot and thinking
that some weird glitch happened that time that would be
fixed by simply running the snapshot again. By adding a
"Re-take snapshot button" we address those cases.
Merge pull request #56 from hellobelda/retake_snapshot

Add "Re-take snapshot" button