add support for custom delimiters in map/slices

bsdlp commented 3 years ago

hey @kelseyhightower @teepark i think this would be a pretty valuable addition to this library (which i use a ton, thank you so much for your hard work).

use case for me personally is setting a env config that's a map of service name to host:port. with existing map support the delimiter : would cause this use case to not be supported

export SERVERS="hypixel;mc.hypixel.net:25565,sep;mc.sep.gg:25565"

type Config struct {
    DiscordToken        string   `required:"true"`
    DiscordWebhookId    string   `required:"true"`
    DiscordWebhookToken string   `required:"true"`
    ImgurClientId       string   `required:"true"`
    Servers             map[string]string `kv_delimiter:";" required:"true"`
}