Limit word count by time period

Issue Summary

I think would we nice if wordcount allows us to count only comments and nodes inside a timeframe. I customiced my module to count last 3 months contributions by adding:

... AND n.created > " . $timelimit

and

... AND c.timestamp > " . $timelimit

to the post and comments SQL select queries inside wordcount_show_count() function, and manually setting $timelimit to desired timestamp BUT, I think would be better if we have a two date fields in wordcount report: "from" custom date and "to" custom date (default now), to limit counting timeframe.