Enhanced mix test --cover

* Generates an `index.html` with summary and links for all files
* Enhance the formatting of the progress lines to include times
* Print a summary coverage line at the end
* Supports `--merge` to merge the multiple HTML files generated for the same source file into a single definitive coverage file
* Supports `--lines` to track un/covered lines using `# [ ...` and `# ] ...` directives

The code isn't the most elegant it could be, but it is straight-forward and effective.