Allow directories to be ignored with -I/IPattern

Currently, the `-I` flag and `IPattern` option only ignore _files_ with the given pattern. Directories matching the pattern remain in the tree.

Would it be possible to also ignore directories? I know this would break the existing API, so I'm not sure if there should be a new option for this, rather than change the existing behaviour.

The `tree` command installed with Homebrew from http://mama.indstate.edu/users/ice/tree/ does ignore directories in addition to files, as shown below:

``` bash session
$ mkdir -p rootdir/building/flowers rootdir/{road,ocean}
$ touch rootdir/building/b.txt rootdir/building/flowers/f.txt rootdir/road/r.txt rootdir/ocean/o.txt
$ tree rootdir/
rootdir/
├── building
│   ├── b.txt
│   └── flowers
│   └── f.txt
├── ocean
│   └── o.txt
└── road
└── r.txt

4 directories, 4 files
$ $GOPATH/src/github.com/a8m/tree/tree rootdir/
rootdir/
├── building
│ ├── b.txt
│ └── flowers
│ └── f.txt
├── ocean
│ └── o.txt
└── road
└── r.txt

4 directories, 4 files
$ tree -I building rootdir/
rootdir/
├── ocean
│   └── o.txt
└── road
└── r.txt

2 directories, 2 files
$ $GOPATH/src/github.com/a8m/tree/tree -I building rootdir/
rootdir/
├── building
│ ├── b.txt
│ └── flowers
│ └── f.txt
├── ocean
│ └── o.txt
└── road
└── r.txt

4 directories, 4 files
```

I'd be happy to work on this, but I'm not sure if it's something you want to add to the project. If so, I'd appreciate some guidance about how to handle backward compatibility.

Thanks!