Support populating slice fields from environment variables

[mark-adams]

Currently, environment variables can only be populated from basic primitives (int, float, string, bool, etc.). It'd be really useful if a user could take a comma-separated value given via an environment variable and populate it into a slice field on a struct.

For example, given the following struct:

type Specification struct {
    AdminUsers                   []string
    MagicNumbers                 []int

a user should be able to set ADMINUSERS = "john,joe,matt" in the environment and have the values populated into the Specification.AdminUsers field on the above struct as []string{"john", "joe", "matt"}

[krhubert]

checkout my repo, I wrote something for my project becouse I need this functionality, you can use it if you want https://github.com/krhubert/envconf

[mark-adams]

Thanks @krhubert! Hopefully @kelseyhightower will merge my PR here as well.