Allow Multiple Installed Versions

[dsanders11]

Currently fiddle-core continually swaps the installed version (via Installer) into Paths.electronInstall as needed. Fiddle can have multiple windows, each with a different version of Electron selected. These two paradigms are at odds with each other since running two different fiddles with different Electron versions will cause the second run to try to replace the files of the first one.

The current approach has the distinct advantage that there's only ever one installed version extracted on disk, so even if no cleanup is performed, at worse the files for a single version remain extracted. The limitation on multiple installed versions is somewhat problematic, though. A better default behavior may be to use temp directories and have Runner remove them as clean up. This would be a breaking change.