Add live reload option to firebase serve

[ninachaubal]

Superstatic already supports live reload, which is awesome for development. This adds it as an option for firebase serve.

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[ninachaubal]

@googlebot I signed the CLA.

[googlebot]

CLAs look good, thanks!

[mbleigh]

@ninachaubal thanks for the PR! We're currently evaluating some of the internals of Superstatic and what we can do to make sure it's a lean, purpose-built library. It's possible (not decided yet) that livereload will be pulled back out with the understanding that it'll be easy to add back in via middleware.

As such, I'm going to close this PR, but it is very much appreciated and I'll count this as a vote for figuring out a smooth livereload process for the Firebase CLI!