expand from default values

[kriptor]

Hi,

for example, if you have the following struct

type config struct {
	Home       string   `env:"MY_HOME"            envDefault:"/tmp"`
	TempFolder string   `env:"TEMP_FOLDER,expand" envDefault:"${MY_HOME}/temp"`
}

and there are no environment variables when you start the process. It would still be nice if you had an option to get Home: "/tmp" and TempFolder: "/tmp/temp", wouldn't it? I mean, we did specify what the defaults are :)

If expand shouldn't be used like that, maybe expandWithDefaults could be.

Cheers!

[caarlos0]

happy to review a PR for that :)