Allow plugin custom commands to be disabled from the plugin config

[JesseTatasciore]

We want to be able to enable / disable certain custom commands from the plugin config. A case where this might be useful... PluginA implements custom commands: Foo Bar

PluginB implements custom commands: Foo Baz

These 2 plugins would conflict with each other unless I was able to disable Foo on PluginB for example

[alexeagle]

maybe for 1.0 we just error when two enabled plugins register colliding commands