Currently, base32 and base64 ignore carriage returns and linefeeds by default.

This behavior goes against RFC 4648, sections 3.3 which state:

Implementations MUST reject the encoded data if it
contains characters outside the base alphabet when interpreting base-encoded data,
unless the specification referring to this document explicitly states otherwise.
Such specifications may instead state, as MIME does, that
characters outside the base encoding alphabet should
simply be ignored when interpreting data ("be liberal in what you accept").
Note that this means that any adjacent carriage return/line feed (CRLF) characters
constitute "non-alphabet characters" and are ignored.

Rejection of "characters outside the base encoding alphabet" (including carriage returns and line feeds) should be the default,
unless specified otherwise by some higher-level specification (e.g., MIME).
The decision to allow \r or \n should not have been made by the base32 and base64 packages,
but rather by the users of it.

Today, base32 and base64 already ignore \r and \n by default and we can't change that,
but we should expose control over this behavior:

// RejectLineFeedscreates a new encoding identical to enc except that
// rejects the presence of carriage returns and line feeds as
// described in RFC 4648, sections 3.1 and 3.3.
func (enc Encoding) RejectLineFeeds() *Encoding