Feature Request: HTML output minor improvements

Please provide the following details if possible or relevant:

- [1.0.0] the version of Ace impacted by the issue (as returned by the `ace -v` command).
- [v8.6.0] the version of Node used to run Ace (as returned by the `node -v` command).
- [Mac 10.13.2 ] your operating system and version.
- [] if the issue is a bug when running Ace: the error message as produced in the log file (you can enable debug logging with the `--verbose` option).
- [Safari Version 11.0.2 ] if the issue is about the HTML report: the browser(s) and version(s) you used.
- [] if possible, let us know if you can share a sample input file to help us reproduce the issue.

Add a space in the comma separated list of metadata properties

schema:accessibilityFeature | tableOfContents,alternativeText,printPageNumbers,readingOrder,structuralNavigation
-- | --

Minor Nit in my opinion: On Images Tab, do not center the alt-text description, or any of the other descriptions for that matter

Reproduce the HTML report, including the metadata property list and Images tab, in Safari 11.0.2 and inspect the generated output. Done means comma-separated metadata values include spaces and image and other descriptions are no longer centered.

Written by the indexing model from the issue text.