[Feature] Get slice values from different env keys

[vejed]

Add iteratesuffix configuration param for decoding slice from separate env values

Example:

type MyStruct struct {
  MyVar []string `env:"MYVAR, iteratesuffix"`
}

export MYVAR0="a"
export MYVAR1="b"
export MYVAR2="c"
export MYVAR3="d"

will substitute values to slice as

MyStruct {
  MyVar: []string{"a", "b", "c", "d"},
}

[sethvargo]

Why? What's the use case?