Feature Request: Add support for altchars

First, thanks for a cool package!

The core function that this package turns into a streaming interface is `base64.b64encode(s, altchars=None)`, and that function supports alternate characters for `+` and `/`:

> Optional altchars must be a bytes-like object of at least length 2 (additional characters are ignored) which specifies an alternative alphabet for the + and / characters. This allows an application to e.g. generate URL or filesystem safe Base64 strings. The default is None, for which the standard Base64 alphabet is used.

Seems like it would be fairly simple to support alt chars in this package by adding a new, optional parameter to the `Base64IO` constructor.

Happy to make a PR to add the feature if it's something the maintainers would want.

Start by locating the Base64IO constructor and the streaming paths that wrap Python's base64.b64encode(s, altchars=None). Trace both encoding and decoding behavior before deciding how the optional parameter should flow. Done means Base64IO accepts alternate characters for + and / while preserving the default behavior; add or update the relevant tests if they are present.

Written by the indexing model from the issue text.