Allow DON format keys to set and get values

Conside the following DON.

    india.capital: New Delhi
    india.demonym: Indian
    india.driving: Left

When this is parsed into a root object, say `root`, we can
access 'New Delhi' in the following ways:

    root.india.capital
    root.india['capital']
    root['india']['capital']
    root['india'].capital

In this change, the following way of accessing the value is
being introduced.

    root['india.capital']

In this new way, the key is specified in DON format.