**:warning: Breaking!** Envconfig no longer runs decoders on unset values! To restore the old behavior, add the `decodeunset` struct field annotation or pass the `DefaultDecodeUnset` configuration option as `true`.

Prior to this change, envconfig would always call decoding functions, even for unset or empty values. This proved problematic for some libraries like `url` and `zap` which implement `TextUnmarshaller`, but error on the empty string (#61). #62 changed the behavior to only call the decoder if a value was explicitly provided, but that broke users unexpectedly (#64), so the change was reverted.

After much discussion, we decided to keep the existing behavior until the 1.0 release. However, after further reflection, I think we need to support a user-level configurable option. Some fields and structs may still want the decoder to run on empty values.

This PR changes envconfig to only process a field if any of the following are true:

- A value was provided (the value can be set to the empty string, there is a distinction between "unset" and "set to empty")
- A default value was provided
- The `decodeunset` struct field tag is set
- The `DefaultDecodeUnset` configuration option is true