Reserve bevy_cli

[DJMcNab]

bevyengine/bevy#436

In theory, a minimum viable version of the cli would be one which simply calls out to external binaries of the form bevy_<subcommand>. For example, I could forsee a bevy_web subcommand which does all of the web stuff.

However, making that happen is development effort, so simply reserving it here is good enough for now.

[cart]

Good call. Done!