Add a plugin for determining disk usage.

This plugin utilises the work in gh-2919 to provide a summary of disk usage by object type:
```
bin/omero usage Image:1,2,3  --report
Using session b011dfc4-2d21-4898-b72b-0a366891414a (user-0@localhost:4064). Idle timeout: 10.0 min. Current group: pg-0
Total disk usage: 153629 bytes
 component    | size  
--------------+-------
 Job          | 59124 
 Annotation   | 8     
 FilesetEntry | 90432 
 Thumbnail    | 4065  
(4 rows)
```
This is very much work in progress in both the name, `usage` is not that good, the object arguments and the output styles, more for discussion than review at this stage.

/cc @mtbc @joshmoore @sbesson

--depends-on #2919
This method is illustrative. If a useful addition then it should probably be 1024 and KiB etc. It may make more sense to truncate rather than show in dps in that case. Additionally I have only applied this to the total as the breakdown is likely to vary in magnitude and so a single set of units would make little sense. Perhaps a "most sensible" scale could be an option? Or the default.