Add Support For Flipping Fuses

Would be nice if `@electron/fiddle-core` could flip fuses before running a fiddle. This would help make it easier to create repro cases for issues which involve fuses for use in Fiddle, and would also make it easier for users to play with fuses with a friendly UX in Fiddle.

I think this would dovetail with the suggested solution in #87 of extracting Electron to a temp directory for each run, so that we can flip fuses without causing side effects between runs.