Provide a mechanism for the server to define what client side features should be present

As we now add support for Mercator mods, we'll get into the situation where any older client connecting to a newer version of cyphesis where there are mods applied will get a terrain that is different from the one defined on the server (since older Ember clients won't understand the "terrainmod" attribute). This might make the experience a little bit jarring, and I'm thinking of perhaps adding some kind of mechanism for allowing the server to specify a series of "features". Any client which doesn't implement any of the "features" should then be able to tell the user this.
I'm thinking of a simple mechanism, where "features" are just specified by simple strings, such as "mercator_terrainmod_1_0" or similiar. This should just be something that the server sends, and it's then up to the client to determine what to do (just show a warning or actively disconnect if not all features are supported).

As "features" I'm thinking of anything that will make the client not represent the world in the correct way. The way we designed the world so far has prevented most of these "breaking" additions, but I'm sure we'll see some more as development proceeds.
If we implement this we'll however have to be quite careful about what "features" we specify. We should only define those things that are actively breaking, not those many cases where older clients just doesn't show as much.