Replayability for explorers

[vinayg-usc]

1. General Idea: Performance metric type thing. We can have our own twist on it by just showing explorable parts of the levels and the progress the player has made on it.
2. Random spawning of gems throughout levels
3. Hidden gems in levels
4. Gems unlock only after completing levels

[vinayg-usc]

• [x] Not clear what the ? is?
• [ ] When to use the powerup, what exactly does it do? Tutorial for it.
• [ ] Move the player back to level select or show a text popup when you get all parts.
• [x] Level 7-1, reduce the delay for the drop.
• [ ] ~Level 5-2, should we make the player a little bit more visible?~
• [ ] Should "Free Mode" unlock the secret? Maybe not, it should be a real secret!

[vinayg-usc]

Closed via #184