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.
