heatmap.bin cutoff

It would be cool if heatmap.bin could be restricted to the top N most abundant OTU. Currently it is hard to get much from it when there are thousands of OTU.

Also I think the group labels could be scaled larger. I made an image with 12 groups and converted it to a 700px-wide
png for web display, at which point the group labels became microdots, although there was plenty of room above each group
column to show them.

Thanks for the suggestion. In version 1.13.0, we are adding fontsize and numotu parameters. We are also adding more sorting options.