Replaces underscore references with native one

[Nandika-A]

Resolves #158

Underscore references in certain files where possible are replaced with native ones as referenced in https://github.com/you-dont-need/You-Dont-Need-Lodash-Underscore. Currently, the native version isn't available for _.any() and _.isEqual() in lines 69 and 70 in lib/subscription.js, so these are skipped for now. The following changes are made-

1. _.each(list, function) to Object.entries(object).forEach(function)
   Nandika-A@da63cfa
2. _.extend(destination, *sources) to Object.assign(destination, *sources)
   Nandika-A@6d5f2a0
   Nandika-A@da3aff7
3. _.keys(object) to Object.keys(object)
   Nandika-A@6d5f2a0

The changes are made in a separate branch Replaces underscore references with native one.
The remaining underscore functions will be tackled in other pr.

[StorytellerCZ]

Great first step. Still more underscore remains, but as I described in the issue that can be taken care of in another PR.