Option for 7-bit ASCII output regardless of input type

[allenap]

I've had an unwritten goal to always produce 7-bit ASCII.

Recent changes to Sh have unavoidably broken that because Dash has no escape syntax for high bytes. Pure binary data quoted/escaped by Sh must still be treated as (mostly) pure binary data – it is not safe to convert quoted output produced by Sh into &str or String for example – but the quoted output from Bash or Fish is safe to treat as text; it's always UTF-8.

Indeed, recent changes in #21 have also broken the 7-bit ASCII rule for Bash and Fish because UTF-8 for code points of U+0080 and above is copied verbatim into the output. It's possible to stick to 7-bit ASCII by encoding text into a byte-like form, like &[u8], before quoting, but this API has a sense of indirectness, action at a distance.

It could be useful to have an option to ask for 7-bit ASCII output regardless of the input.

In any case, this crate should state the 7-bit ASCII goal explicitly in documentation, and back it up with explicit tests.