Allow users to group expvars into custom TermUI lists

[Zachary Gramana]

Would be nice to be able to group panels like this:

    ./expvarmon -vars="foo,bar"  -vars="fizz,buzz"

or even

    ./expvarmon -vars="Memory::foo,bar"  -vars="CPU::fizz,buzz" #single colons may collide with expvar names for some