Read gfx.yaml for graphics conversion options.

Keys count both as path nodes and attributes depending on context.

An example gfx.yaml:

```
gfx:
    pics:
        pal_file: normal.pal
        pic_dimensions: yes
        animate: yes
        stupid_bitmask_hack: [dewgong, lugia]
    pc:
        literal_only: yes
```

Here, all graphics matching gfx/pics/.../*.png will use palette gfx/pics/.../normal.pal and be interpreted as animated pics. gfx/pc.2bpp will compress using only the `literal` command.

If gfx.yaml doesn't exist, current behavior is unchanged. Filename arguments override yaml.