Add support for looking up optional pegdown dependency #11

When using a recent version of scalatest the pegdown dependency
is required for htmlreports.  To use htmlreports add the pegdown
dependency to the plugin definition in your project.

  <plugin>
    <groupId>org.scalatest</groupId>
    <artifactId>scalatest-maven-plugin</artifactId>
    <dependencies>
      <dependency>
        <groupId>org.pegdown</groupId>
        <artifactId>pegdown</artifactId>
        <version>1.4.2</version>
      </dependency>
    </dependencies>
  </plugin>
The original problem is only about pegdown dependency for reporting. I think it would be better to have this solution more generic - to work with any dependencies set on the plugin level.