Rework the names for the colored coverage elements

[ryanluker]

Reference

#239 (comment)

Problem

The old names for the coverage elements (highlights) is quite confusing now that we have gutter coverage elements as well as line and map based ones. We can use the example naming by @DevilXD as a starting point for switching to something more sensical.

Work

1. Slightly adjust the settings names to make more sense regarding their visual nature.
2. Figure out if this change should be in the next minor release or next major (changing the setting name will probably mean a breaking change unless we deprecate the old value and use it as the default for folks that have set it)