Handle fuubar

[inkstak]

Fix #23 : Allow to use the Fuubar formatter.

This pull request depends on #41.
The changes from #41 could be seen inkstak/turbo_tests@feature-parallel_options...feature-handle_fuubar

[inkstak]

I had issues when using JSON formatter for extracting the number of total examples as suggested in #23 (comment)
The backticks capture the full output of the command, which may include other puts.

For example:

• We have Dotenv configured to output which files are loaded on top of every rspec command:
  [dotenv] Load .env.test

• Simplecov might output a line after the JSON output :
  Coverage report generated for ... (x%) covered.

Any ideas ?

[inkstak]

PR updated :

• I finally succeed to properly calculate the examples count using the JSON formatter as suggested in Usage with fuubar #23 (comment) by using a temporary file.
• I removed all references specifici to Fuubar: the changes could now benefit to any custom formatter that requires the examples_count.

Changes are splitted in two commits :

1. e1f8f6b is only a refactoring of the Runner class to allow forwarding files & parallel options (which depends on Support parallel_test options: pattern, exclude-pattern & group-by #41) to the Reporter

2. 6bfb97b add a start notification with the examples_count

[inkstak]

One more thing : to use Fuubar, you need to require it in the command :

bundle exec turbo_tests -r fuubar -f Fuubar spec/whatever