Introduce union decoders

@bageren contribution to introduce union decoders for String and Number, example: asStringUnion("dev","qa", "prod")

Use optional decoder with default value

Functionality to add default value for non-required environment flag. Expand the functions as{Type} to
include .withDefault({type}), fx: asBoolean.withDefault(false)

Accumulate decoder errors

Accumulates all decoder errors before throwing (#4).
Update documentation with examples.

Decode environment variables

Decode environment variables to typed object