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`).
Remove build files

This is already in the gitignore file but somehow old copies were still
stored in the history. This cause an issue if the test suite outputted
files that differ from the stored files causing git to complain about
modified files. Since they are to be ignored git doesn't need to track
them.
Merge branch 'cleanup-build-files'