Command line editor for ion

Created by: skylerberg

Shells like bash and zsh have line editors that make them far more usable. For example here are some features provided by GNU Readline used in bash:

• Tab completion
• Up arrow for previous command
• Reverse search (ctrl-r)

Readline is kind of the standard for line readers, but it is under a GPL license. It would be nice to have our own line editor written in Rust that can be used by our shell.

NOTE: This was originally an issue opened in Redox (https://github.com/redox-os/redox/issues/346), but I am moving it here.