feat(dataHub): adds countries and cities population features

[MartinsOnuoha]

What's New?

This PR:

• adds an endpoint to get all countries and their population data between (1961) - (2018)
• integrates dataHub JSON data endpoints
• adds an endpoint to get a single country and its population data (1961) - (2018)
• adds an endpoint to filter countries and population data by (year, order, orderBy, lt, gt, limit)
• adds an endpoint to get all cities, related country, and population data
• adds an endpoint to get a city by name, with its population data
• adds an endpoint to filter cities and its population data by (country, order, orderBy, limit)

Closes #9

Note: both commits are intentional. Check Readme for Endpoint Documentation.

[iambenkay]

Good Job!!