Rework Pager formatting

This patch allows the user to mark up workspace names with access to
monadic context.  This enables things like,

```haskell
    myMarkupWorkspaces :: Traversable f => f WorkspaceInfo -> f Markup
    myMarkupWorkspaces wss = evalState (traverse f wss) ("", cycle colors)
      where
        colors = [ "#843c3c"
                 , "#60843c"
                 , "#3c8384"
                 , "#3c4d84"
                 , "#613c84"
                 , "#843c3c"
                 ]

        f :: WorkspaceInfo -> State (String, [String]) Markup
        f ws@(WSInfo {wsiName=name}) = do
          let group = takeWhile (/= '-') name
          (lastGroup, grpColor:rest) <- get
          if group /= lastGroup
            then put (group, rest) >> f ws
            else return $ color grpColor ws

        color grpColor (WSInfo {wsiName=name}) = colorize grpColor "" $ escape name
```

Which colors workspace names in groups according to their prefix
(delimited by '-').