.mailmap: Standardize identifiers for Lynne and Matt
.mailmap: standardize identifiers for Joshua, Katy, Matt, and Sri
.mailmap: Flesh out with Aron, Cath, Christina, Lex, Molly, Scott, ...

... Steve, Trent, and Yuxi.

The previous two commits came from swcarpentry/boot-camps#9 and
swcarpentry/boot-camps#24.  This adds entries for other authors, as
far as I could determine from searching the GitHub and the wider net.
Merge pull request #288 from wking/mailmap-from-boot-camps-repo

Mailmap from boot camps repo
Merge branch 'master' into v5alpha
Adding target to Makefile to list contributors
Adding list of contributors