Support for Bullet Charts

This addition adds support for bullet charts. The renderBulletChart function expects an expression that will return a named list containing elements named ranges and measures at the minimum.  Optional elements named title, subtitle, markers, rangeLabels, measureLabels, and markerLabels are allowed.

Some general notes regarding reactivity in the Shiny context:
- Title and Subtitle are non-responsive.  They should not be set to the values of textInput widgets as they will not update when the test is edited.
- RangeLabels are reactive, but measureLabels and markerLabels are not.
- Marker plotting can be toggled off from the on state, but not on from the off state.  To toggle off, set the markers element to null.
- The color of the measure (the bar) can be set via the cols element.  R color names, rgb vectors and hex codes are all viable color options.
- Range values, the measures value, and the markers value are all responsive values.
Minor Edits to documentation and field values

Changed the starting value of max field to Maximum from Minimum.

Corrected some wording in the README file.