Remove flat string input

So far only `[]string` inputs have been used, which seems reasonable as the sole accepted input type. Users can just use `strings.Split(text, "\n")` if they want the flat text input functionality.

The reason for this change is to allow us to have an absolute type in `Format()` and `SimpleFormat()` function signatures, which will pull argument errors up to compile time rather than spewing them at runtime if an invalid argument is passed.