Force to ground

New features

Add a keybinding ("g") to force the ruler to measure from ground terrain. Replaces "Prefer Token Elevation," which was removed. Closes #63, #64.

Bug fixes

Catch when a segment color is not defined, to avoid throwing an error.
Fix for incorrect combat speed movement highlighting after the first move. Closes #62.

Full Changelog: 0.8.10...0.8.11