Support for reading/writing container metadata

Adds the `:clear_existing_metadata` option which, when set to true,
will add flags to ffmpeg to remove existing metadata on the media
container.

Adds the `:metadata` option which accepts either a **String** containing
the `-metadata` argument you wish to set on the container, or a **Hash**
of key-value pairs which will be set on the container.

```ruby
movie = FFMPEG::Movie.new('path_to/movie.mov')
movie.transcode('path_to/movie.mp4', {
  clear_existing_metadata: true,
  metadata: {
    creation_time: Time.local(2005, 03, 04, 14, 30, 00),
    'Application' => 'SomeApp 1.0'
  }
})
```

**...or...**

```ruby
movie = FFMPEG::Movie.new('path_to/movie.mov')
movie.transcode('path_to/movie.mp4', {
  clear_existing_metadata: true,
  metadata: 'Application="SomeApp 1.0"'
})
```

Tests included.

Not sure if anyone else would find this useful, but figured I might as well contribute.