Support colon in map values

[UnAfraid]

Since colon character ':' is being used as key-value delimiter that might cause problems when you want to have a map of name=url By using SplitN we can grab the first : which should be the end of the key name and allow having one or more colons within the value without any issues.

[jasonfriedland]

I was about to raise this very change; +1 please consider this to support URL values in map items.

[hori-ryota]

duplicated.

Replace Split to SplitN for contains colon value map by hori-ryota · Pull Request #133 · kelseyhightower/envconfig

[prusnak]

go-envconfig can override separator via separator keyword: https://github.com/sethvargo/go-envconfig#separator