feat(preserveAspectRatio): added option to preserve aspect ratio

When set to true and both width and height are specified, they are interpreted as the minimum width and minimum height, respectively.
If set to true with only the width specified, the height will be automatically determined while preserving the aspect ratio, and vice versa.

fixes #88, #189

src/graphics.ts

@@ -39,7 +39,6 @@ export class Graphics {
   }
 
   public gmBaseCommand(stream: fs.ReadStream, filename: string): gm.State {
-    // console.log(JSON.stringify({ width: this.width, height: this.height, preserveAspectRatio: this.preserveAspectRatio, density: this.density }))
     return this.gm(stream, filename)
       .density(this.density, this.density)
       .resize(this.width, this.height, this.preserveAspectRatio ? '^' : '!')