Expose gm option to support gm.exe in PowerShell

[jasonhargrove]

I work in Linux, Mac, and (ahem) Windows (haha), and happen to be on Windows now. In PowerShell pdf2image throws an error because the gm maps to something else. In PowerShell gm.exe is the command.

For now I have hacked the gm module like this for Windows.

GraphicsMagick offers an appPath argument (see screenshot).

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

via aheckmann/gm#681

It may be something worth supporting in your config.

Unfortunately it won't work in PowerShell without a hack as is. I'm not 100% sure what I'll do as I'm just testing the module. Got it to work now with this hack. Let's see how it goes