FIX: Hyperdispatch.from in normal usage does not set { offset } in opts

[ryanramage]

Running the example.js file twice would throw the following error

/Users/ryan/github/holepunch/hyperdispatch/builder.cjs:27 throw new Error('Cannot change the hyperdispatch offset once it has been defined once') ^
This was because the offset in the json was 0, but the opts when using the Hyperdispatch.from would be undefined in normal world usage.

This fix

• makes sure that offset is defined before doing the offest comparison
• runs example.js twice in normal usage as part of the npm test to ensure normal rebuilding works
• excludes the spec folder from git

[andrewosh]

Nice LGTM