Add main file to import into JS

[NJRBailey]

Pull Request following discussion at #12

I was wrong in my initial issue - I couldn't get it to work in the browser as easily (I actually couldn't find a way of getting web browsers to open the JSON directly either, so it may not be possible to use the library in browser even in its current state).

This PR adds methods to synchronously load the JSON. There is a way of doing it asynchronously, but it is still an experimental part of the JS spec, so I did not include it.

Using the code

import geoJson from "world-geojson";

geoJson.forCountry("Antigua & Barbuda"); // Converts to "antigua_and_barbuda"
geoJson.forArea("U.S.A.", "U.S. Virgin Islands"); // Converts to "usa", "us_virgin_islands"
geoJson.forState("Australia", "New South Wales"); // Converts to "australia", "new_south_wales"
geoJson.forCountry("abcd"); // returns `null` when data not found

Filename conversion

The name conversion is very simple - users can generally specify the English names of places and the conversion will successfully match the filename:

name
    .replace(/ /g, '_') // Replace spaces with underscores
    .replace(/\./g, '') // Remove dots
    .replace(/&/g, 'and') // Replace & with and
    .toLowerCase() // Make lowercase

They can also specify the filename directly and it would still work:

geoJson.forCountry("antigua_and_barbuda");

I can make this more helpful if you want (for example, adding support for strings like 'United States of America' or 'Democratic Republic of the Congo').

Tests

I've manually tested a range of countries, states and areas.

I have not added any automated tests - I am happy to add some but it would require me to add a test library dependency.

[georgique]

Looks good, thank you