Tweak comparer styling
Add attribute value formatting

The compare feature plugin previously only accepted field names for the
attributes to display, and used the published ArcGIS alias for
rendering.
This commit lets the admin specify more detailed formatting options:

* format types: number, percent, text
* numeric options include rounding, units & a threshold to prevent
values
  being rounded to 0
* `text` can truncate characters and provide default "no value" labels
* Raw values are visible in the tooltip
* Config errors are logged to the plugin log, but raw values will be
shown

These are specific formatting features requested for Compare Watershed
Merge pull request #209 from mmcfarland/topic/format-comp-features

Formatting for Compare plugin attribute values