Allow users to group expvars into custom TermUI lists

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

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

   or even

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