This small crate implements Windows-style and Unix-style shell escaping.
Its not clear which exactly are the supported shells, so its hard
to guarantee the escaping is safe for all possible ones (although it
looks fine for all common shells).

A second worry is the runtime detection of the current shell, where an
environment variabe (MSYSTEM) can be used to change the escaping style to
Unix. For users of the crate, if the environment is known, I would recommend
explicitly specifying it, instead of relying on the detection.

The exposed functions have the signature Cow -> Cow. It would seem
that &str -> Cow is more natural.

I do not know if character encodings play a role, here, and if str is the
correct type to be working with.