Grunt:Allow different report location and filename

Allow different report location and report filename when using the reporter with Grunt:
Currently "mocha.js" report is always written into the root folder of the project.

Example usage with Grunt & Gruntfile.js:

mocha: {
  test: {
    src: ['src/test/frontend/*.html'],
    options: {
      run: false, //run in PhantomJS
      reporter: './node_modules/mocha-bamboo-reporter'
    },
    dest: 'target/surefire-reports/mocha.json',
  }
}