The goal for these examples is to show how to mirror the
functionality of the sha256sum Unix utility, a common checksumming
tool, using the Go standard library.
Add a newline at the end of the input, so users will get the same
output if they type `echo 'hello world' | sha256sum`, since the
builtin shell echo appends a newline by default. Also use hex output
(instead of the shorter base64) since this is the default output
encoding for shasum/sha256sum.