Multiple reports

[dannylewis-sheffield]

Added the option to allow multiple reports.

Sample behat.yml:

        format:
            - html
            - clover
        output:
            html:
                target: public/coverage-behat
            clover:
                target: clover/behat.xml

I've left the original report option as is, so no breaking changes.

[ek9]

Hi,

Thank you for the PR. There are a few problems:

1. The code update is very messy in ReportService.php. The nested if/then/else is unclean and there should be a more clean way to handle this.
2. The code style is very different from the current coding style that project is using. I have made Add conding style guide and checker #25 so we have code style checks as part of the Travis.
3. The travis build for your update contains errors (the tests fail). The tests must be fixed.