Add cmdline option to load build settings from file

For cmdline users of the arduino-builder it would be nice to run the tool with custom build settings read from a file. 

```
arduino-builder --pref-file ~/Whereever/my.standard.build.options.json aSketchToCompile
```

BTW: If the properties in the current build.options.json file were delimted with newline `\n` chars, that file would make a good template for custom settings. 

Eberhard
