Provide option to customize http headers

[luksak]

Since some Novatim instances require you to provide a custom referer and user agent, we should provide the option to customize http headers. This PR enables top set them like this:

const Nominatim = require('nominatim-geocoder')

const geocoder = new Nominatim({
  delay: 1000,
  headers: {
    Referer: 'https://www.example.org/',
    'User-Agent': 'My geocoder script',
  },
})