Add option to enable content hashing when in watch mode

[joeosburn]

↪️ Pull Request

This adds the ability to enable content hashing for watch mode. It is the inverse of --no-content-hash. This is useful if you want content hashing enabled when in development mode. See #188 and #1025 for reference.

💻 Examples

parcel watch app/entry.html --content-hash

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[shanebo]

@DeMoorJasper, this is badly needed. I hope y'all consider merging this simple flag in.

[DeMoorJasper]

@shanebo this PR edits Parcel v1 code, which we're not planning to update anymore especially not with new features.

Parcel 2 is getting very close to a beta release, in Parcel 2 this will be possible by adding a namer plugin.

I hate to do this but I'll close this PR as it would probably never get released.