Option to enforce datatype + bug fix

• You can now enforce datatype of environment variable or default value with tag:yaml.org,2002:datatype e.g. tag:yaml.org,2002:float (workaround because !!float is not allowed when there is an !ENV tag. (see #26 )
• Bug fix: parse only environment variables that are denoted with !ENV tag, not any environment variable (see #25 )