Reformat Client list into per-language sections

The Client list was getting unwieldy and difficult to navigate. This pull
request reformats the list into per-language sections. Additionally, a new
section has been created (Health and Performance Monitoring), which now
includes plugins like bigdesk, SPM and nagios-related.  This moves them
out of Misc. which is less descriptive.

Lastly, these have been added:

- es2unix
- dangle (closes #379)
- SegmentSpy
- check_elasticsearch and opsview-elasticsearch (closes #372)
- Inquisitor
- Sherlock