add scree plot to ordination panel
Format scree-plot positioning on Ordination panel

Completes feature in
https://github.com/joey711/shiny-phyloseq/issues/22
Merge branch 'add-scree-plot'
Add original/filter option to alpha-diversity panel

This addresses:

https://github.com/joey711/shiny-phyloseq/issues/24

Default is now `Original` (unfiltered) data. This is most appropriate
for the alpha diversity estimates in most scenarios.