Add test-code-coverage command - #165

This PR adds a command ember test-code-coverage. This can be used e.g. in CI environments.

The command can also be used to simply print out the total coverage percentages, e.g. so you don't need to go looking into HTML files for them.

It takes the following parameters:

• target-lines: The target percentage for lines covered
• target-functions: The target percentage for functions covered
• target-statements: The target percentage for statements covered
• target-branches: The target percentage for branches covered

For example: ember test-code-coverage -target-lines=80 -target-branches=85

You can also configure those in the config/coverage.js file, in their camel cased form, like this:

 module.exports = {
    targetLines: 80,
    targetFunctions: 85.5,
  
    // Other configuration
    coverageEnvVar: 'COV'
  }

These values will then be the defaults when running ember test-code-coverage.

Running the command will output something like this to your console:

+ Lines covered:       60.00%   69.17%
+ Functions covered:   --.--%   72.18%
+ Statements covered:  --.--%   68.93%
- Branches covered:    70.00%   60.16%
- Test coverage check failed

For any type of check where no target is specified, it will simply output the actual value without doing any comparison.