Support Multiple Buttons in the .on Method

similar to #2, it would be helpful to support multiple buttons in the `.on` method by sending an array as the first arg:

``` js
// multiple buttons
mpr121.on([2, 3], () => console.log('pins 2 & 3 were pressed'));
```

Start by locating the library's `.on` entry point and review issue #2 for the related behavior. Done means the first argument accepts an array such as `[2, 3]`, the callback responds to either listed button, and the existing single-button form still works.

Written by the indexing model from the issue text.