Added CLI

```
$ gify

  Usage: gify [options] infile outfile

  Options:

    -h, --help               output usage information
    -V, --version            output the version number
    -w, --width [width]      max width [500]
    -h, --height [height]    max height [none]
    -d, --delay [delay]      delay between frames [auto]
    -r, --rate [rate]        frame rate [10]
    -s, --start [time]       start position in seconds or hh:mm:ss[.xxx] format [0]
    -D, --duration [length]  length of video to convert in seconds or hh:mm:ss[.xxx] format [10]
```