To close #2

[cool-Blue]

closes #2

Proposed solution is to wrap the worker's terminate method and revoke the blob object URL there.

[andyearnshaw]

Hey, thanks for the PR and apologies for not spotting it in the last few months. Would it be more appropriate to revoke the object URL after the worker has been initialised rather than doing it on terminate?

I'm thinking that many workers aren't terminated until the page unloads and, while the memory usage isn't a big deal I'd prefer to remain as efficient as possible.