A little function that reads an environment variable from
process.env with support for default values and a optional mapping
function applied to the data read from the environment.
It will thrown an exception if key can't be found and no default value is supplied.
var getEnvVar = ;;;;
The following shorthand methods currently exists. They have the same API as the generic method. The mapping function is applied to the transformed value.
getEnvVar.boolean is a shorthand method that returns true if the
value equals 'true' and false in all other cases.
getEnvVar.integer is a shorthand method that calls