Fixes #258.

http://trac.haskell.org/haddock/ticket/258

Example output:

```
  54% ( 21 / 39) in 'Documentation.Haddock'
  Missing documentation for:
    Haddock.Types.ExportItem:175
    Haddock.Types.DocForDecl:237
    Haddock.Types.Doc:297
    Haddock.Types.Example:363
    Haddock.Types.Hyperlink:353
    Haddock.Types.DocMarkup:378
    Haddock.Types.Documentation:223
    Haddock.Types.ArgMap:43
    Haddock.Types.AliasMap:454
    Haddock.Types.WarningMap:122
    Haddock.Types.DocMap:42
    Haddock.Types.HaddockModInfo:401
    Haddock.Utils.markup:415
    Haddock.InterfaceFile.InterfaceFile:47
    Haddock.InterfaceFile.nameCacheFromGhc:162
    Haddock.InterfaceFile.freshNameCache:173
    Haddock.InterfaceFile.NameCacheAccessor:159
    Haddock.Options.Flag:42
```

Enabled with `--print-missing-docs`. We don't print the column number because Haddock will only nag us about things that are always at column 1 (or at least I can't think of a situation where it will be different and matter).

We drop the module name where we can so we get

```
  25% (  1 /  4) in 'Uni_Module'
  Missing documentation for:
    Module header
    Foo:8:1
    […]
```

and not
```
  25% (  1 /  4) in 'Uni_Module'
  Missing documentation for:
    Module header
    Uni_Module.Foo:8:1
    […]
```