Add options for specifying output dir and dealing with non-xxxhdpi inputs

Adds the following options.

```
  --scale-hint-horizontal SCALE_HINT_HORIZONTAL
                        Calculates scales by assuming the provided input image
                        spans the full width of a xxxhdpi/retina screen and
                        applying the same scale to all other inputs.
  --scale-hint-vertical SCALE_HINT_VERTICAL
                        Calculates scales by assuming the provided input image
                        spans the full height of a xxxhdpi/retina screen and
                        applying the same scale to all other inputs.
  --android-input-density {mdpi,hdpi,xhdpi,xxhdpi}
                        Assumes inputs are sized for the specified density.
  -o OUTPUT_PATH, --output OUTPUT_PATH
                        Outputs to the provided foler.
```