make _show a bit easier to read

This PR adds a bit of functionality when showing groups.

1. For lists and tuples longer that 6 members, it prints an abbreviation of the collection along with the number of elements
2. It adds some color, making it easier (at least for me) to read.

I may not have implemented either of these to your liking.  The coloration, in particular, is obviously flawed.  The hard-wired colors would be unreadable on a terminal with a light background.

The collection abbreviation corrects an aspect of my feff85exafs unit testing tool that has been annoting me.