3.5: Fix :HighlightCoverageForAll when .coverage is not in $PWD

• Pay attention to &wrapscan (GH: #27)
• Avoid hit-enter prompt when &cmdheight is 1 (GH: #28)
• Document :ToggleCoverage in the README (GH: #29)
• Fix :HighlightCoverageForAll so it can find the files when the .coverage file is not in the current working directory