fix: allow to use parameter resolution in configuration for bootstrap key

[wuchen90]

In PhpStorm, I've got an issue with path resolution for the bootstrap file. I don't know why the path in which Behat runs is always the folder features. So Behat always fails trying to look for /project/root/path/features/config/bootstrap.php.

With this fix, bootstrap file will be resolved based on the base path. Plus, people will be allowed to use the parameter %paths.base% to define a custom bootstrap file path. E.g.:

default:
  extensions:
    FriendsOfBehat\SymfonyExtension:
      bootstrap: '%paths.base%/custom/bootstrap.php'