Better support for dynamically-generated commands?

[recatek]

Currently it seems very difficult to generate commands at runtime (e.g. from reflection on a struct you want to edit fields of). NamedCommand and other similar traits don't take self for example, so you can't read or use values on the command you're configuring as you send it in. You can use the generic Command, but then you can't differentiate by type (e.g. setting a bool vs. setting an int).

It would be handy if bevy_console could relax some of the assumptions that every command is known at compile-time when building.