Add option for raising an error on missing ENV variable

[Brias]

This PR implements #22 by adding a raise_if_missing option to the parse_config function.

If raise_if_missing is set to True, a ValueError is raised when the following conditions are met:

1. The ENV variable is not set
2. No default is set for the ENV variable

Example:

# ValueError is raised
data: !ENV ${NON_EXISTING_ENV}

# No ValueError is raised
data: !ENV ${NON_EXISTING_ENV:explicit_default_value}

For backwards compatibility, raise_if_missing is set to False by default.

[maldoinc]

+1. If anything, should be the default behavior of raise_if_na. Sometimes you just don't have a default value.

[pierec]

+1, very useful!

@mkaranasou - can we get this merged, once you have a spare moment?