Add taint analysis

This release adds two new steps definitions, @Given I have Psalm with taint analysis and @When I run Psalm with taint analysis, which allow you to use taint analysis:

 Scenario: Running with taint analysis
    Given I have Psalm with taint analysis
    And I have the following code
      """
      <?php echo $_GET['param'];
      """
    When I run Psalm with taint analysis
    Then I see these errors
      | Type            | Message |
      | TaintedInput    | /./     |
    And I see no other errors

Features

• (#16) Run Psalm with taint analysis (thanks @muglug)