Add metadata to main data object

There are times when templates might want access to the tags data and not be
one of the tag output files. Currently the tags data is only available in the
output file (`topics/{tag}.html`) for which your template can iterate over the
tags object.

With this feature the user can iterate over tags outside of the output file by
accessing the `metalsmith.data.tags` property (or in the case of a template
`this.tags`).