Support for webassembly and WASI

I'm working on https://github.com/mitnk/cicada/issues/27 (port cicada (a bash-like shell) to WASI). This crate is an indirect dependency of cicada.


Can you add support for WASI (target `wasm32-wasi`)? Your package has been used in many packages and will be very useful for server-side webassembly ecosystem.