Update to Crystal 1.0.0+ and better integrate with spec

[lbguilherme]

This PR includes the following changes:

1. Make it build and collect coverage with Crystal 1.0.0+;

2. Add --help cli flag;

3. When run without specifying filenames, use the spec files inside the spec directory, the same behavior as crystal spec. It can now be used simply as bin/crystal-coverage.

4. Improve console output to better integrate with spec:

   $ ./bin/crystal-coverage
   ....
   Lines 81.93% covered
   
   Finished in 3.0 milliseconds
   4 examples, 0 failures, 0 errors, 0 pending
   

5. Modify HTML output to show line data as a single column. This change is in preparation to also collect branch and function information.