Nested structs handling #1

Solves the nested struct handling. Struct fields can be tagged with `env:"nested"` in order to be loaded in the same function call as their parent, otherwise they will be ignored. Using any other tag from this package will result in an error.

thanks, i've wanted to add this feature but didn't had the motivation. altough the struct tag should be actually useful for the nested syntax.

example of code:

typeConfigstruct{ServerServerConfig`env:"SERVER_"`// this will be the prefix of all env vars for ServerConfigDebugbool`env:"DEBUG,default=false"`}typeServerConfigstruct{Portint`env:"PORT"`// so this will parse as `SERVER_PORT`}

Done 👍

lgtm