Added an image sequence option

With `sequence: true` you can define the source as a directory with a wildcard.

```js
var opts = {
  width: 300,
  sequence: true
};

gify('frames/*', 'out.gif', opts, function(err){
  if (err) throw err;
});
```

Also added this option to the CLI but you need to wrap the source in quotes.

```
$ DEBUG=gify ./bin/gify -S -w 300 ../images/* blah.gif
[ '../images/DSC_0031 2.JPG',
  '../images/DSC_0031 3.JPG',
  '../images/DSC_0031 4.JPG',
  '../images/DSC_0031 5.JPG',
  '../images/DSC_0031 6.JPG',
  '../images/DSC_0031 7.JPG',
  '../images/DSC_0031 8.JPG',
  'blah.gif' ]
```


Maybe it should assume the source is a directory and append the wildcard character if the sequence option is enabled.