`cluster`: develop downstream usage and visualization

[bluegenes]

• optionally output dot file of graph?
• tsne/other cluster visualization
• ref @mr-eyes work with kSpider/dbretina

[ctb]

yes - it would be great to put together some simple/straightforward viz tutorials!

[ctb]

the betterplot plugin now provides the following commands for working with pairwise and cluster output:

• mds2 - MDS plot of pairwise output, with optional coloring by categories
• tsne2 - tSNE plot of pairwise output, with optional coloring by categories
• cluster_to_categories - build categories files from cluster output

See the Snakefile under examples/ in the betterplot github repo for a functioning example!