Add a meta-project for external plugins

External plugins are Fiji plugins whose source lives outside of the
Fiji repository.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Make Sholl Analysis an external plugin

The Sholl Analysis plugin's sources live in

	https://github.com/tferr/ASA

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'sholl-analysis'

This makes the Sholl Analysis plugin an externally-maintained one.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>