Add package-lock.json

[alulsh]

We should add a package-lock.json file to this project to avoid issues like https://github.com/mapbox/dispatch/issues/84 in the future.

Per the npm docs on package-lock.json:

package-lock.json is automatically generated for any operations where npm modifies either the node_modules tree, or package.json. It describes the exact tree that was generated, such that subsequent installs are able to generate identical trees, regardless of intermediate dependency updates.

This file is intended to be committed into source repositories

/cc @oliikit @zmully @k-mahoney

[alulsh]

Added in https://github.com/mapbox/dispatch/pull/94.