Bump autoprefixer version, add option to disable it or provide configuration options

## Summary

Bumps the `autoprefixer` dependency to the current version 3.1.0.

Also introduces the ability to disable `autoprefixer` processing, if desired, by setting

    "_autoprefixer": false

in a `_data.json` file when using `harp`. Alternatively, this key can be used to provide configuration options to `autoprefixer`, such as the [`browsers`](https://github.com/postcss/autoprefixer#browsers) option, i.e.

    "_autoprefixer": {
        "browsers": ["> 1%"]
    }

This fixes #68.

## Caveats

The `_autoprefixer` setting is only available on a per-directory basis, via `_data.json`, NOT in the global `_harp.json`.