Document appPath option

[zackiles]

I had to read the source to find this undocumented option for appPath at https://github.com/aheckmann/gm/blob/master/lib/command.js#L204

This allows someone to specify the gm binary path if it's not in PATH. This was particularly helpful for me when using it on Windows.

import GM from 'gm'
const gm = GM.subClass({appPath: 'C:\\Program Files\\GraphicsMagick-1.3.26-Q8\\'})

[jake-knerr]

Thanks bud, this worked for me and undoubtedly saved me a ton of time.

[Aspvik]

Thank you!