The old method signature was just nonsensical, as you would always be
providing indented values via a string literal. So it makes much more
sense to have all methods accept a string argument, and then return
different types.
This also allows use of a `Bytesf` method.
This is technically a breaking change, but I'm classifying it as a
bugfix cause the old method signature was basically useless.
This effectively cleans up what I consider syntactical sugar required
due to Go's syntax. For example:
str := undent.String(`
hello
world`,
)
In the above example I would consider the initial line-break after the
opening back-tick (`) character syntactical sugar, and hence should be
discarded from the final undented string.
However if the literal string contains more than one initial line-break,
only the first one should be removed, as the rest would intentionally be
part of the input.
