Add ability to load exported palettes, via URL or via the exported file

Load palettes from Photoshop palette file, GIMP palette file, or a plaintext like this:

```
#000000
#fcfcfd
#d0cbc2
#9b7774
#40474e
#cc181d
#362422
#1a1a1a
```

Related info: my SO question about [color palette file formats](http://stackoverflow.com/questions/6281385/is-there-a-standard-xml-or-json-schema-for-color-palettes).
