Save drafts

[chiel]

Basically save the current tinker to localstorage on a regular interval + on every run. This way if someone's browser crashes, the code can be recovered when they revisit Tinker. In the future it would be excellent to allow for multiple drafts at the same time to be stored, as well as drafts for tinkers that already have been saved.

This feature would be more of a failsafe than anything.