Add `enabled` key to config

Archives are enabled based on the following rules:
- All archives are disabled by default
- All archives can be enabled by setting `enabled: true` or `enabled: all`
- Individual archives can be enabled by specifying `enabled` as an array (for example, `enabled: ["tags", "categories"]`

Ref [comment in #2](https://github.com/jekyll/jekyll-archives/issues/2#issuecomment-52978072)