New command: //fillcaves

[sbrl]

When we're using //conv with the intention of using //layers, we sometimes get 'caves' occurring whereby we have a relatively thin 'crust' of nodes above a cavern of air. It would be great to have a command to fill these in with a given node. It could take the following form:

//fillcaves [<node_name>]

Example invocations:

//fillcaves
//fillcaves stone
//fillcaves dirt

Perhaps it could default to stone?