Add caching of reports

[morganestes]

Every report is generated by multiple calls to Trac, then displayed but not stored anywhere.

Let's add some form of caching, whether it's on a per-report basis (caching the response from Trac, then parsing it), saving the generated report to a file or caching system and returning it instead of regenerating, or just the markdown and regenerating the HTML each time (seems silly, though).

Things to consider:

• How to store the data
• What data to store
• How often to flush and regenerate the cache
• How to force a report regeneration by a site visitor