Function to overlay a constants object.
$ npm install consts-overlay
const consts = require('consts-overlay')
const Consts = consts({
FOO: 'foo',
BAR: 'bar',
}).overlay({
FOO: 'foofoo',
BAZ: 'baz' // ignored if a key is not in the original object
})
// {
// FOO: 'foofoo',
// BAR: 'bar'
// }