Stripping spec coverage should be a configurable option

[bahmutov]

As asked in #52 the user wants to have spec file coverage in the report. Right now we strip this information, leaving code coverage for application source code only (see https://github.com/cypress-io/code-coverage/blob/master/support.js#L63). Make it into a configurable option.

[cmelion]

It would be very useful to get coverage reports for step files. IDE integrations make it very easy to determine when step implementations for [Given, When, Then and End] are missing but the converse isn't true and for larger projects it's possible end end up with deprecated or duplicate step implementations.

For Example