Consider adding a UI to exclude certain condition plugins from menu position

[star-szr]

This came up in a brief in-person discussion and relates to #22. Could we have a UI that lists all the condition plugins found with a checkbox for each one, allowing problematic contrib condition plugins to just be disabled by the user? We could also ship with certain defaults, for example not enabling the theme condition by default.

[ckaotik]

The issue of "unwanted plugins" is a general issue that at least I have run into multiple times now. Be it rules' condition plugins or migration temlates or any other, there are always ones you don't need or don't want because they break things or confuse the user.

However, I don't think this is in the scope of menu_position and should probably be a full standalone module, if we can even dig this deep into D8's functionality.