Add exec goal

Add a simple goal for executing phantomjs after it has been downloaded.  While in general this plugin was meant to be used in conjunction with other plugins and tools, this seems like a simple enough feature to add so I think it should be added.

The following example would download phantomjs 1.9.2 and then execute `phantomjs --load-images=false src/main/javascript/example.js arg1 arg2`:

```xml
<plugin>
  <groupId>com.github.klieber</groupId>
  <artifactId>phantomjs-maven-plugin</artifactId>
  <version>${phantomjs-maven-plugin.version}</version>
  <executions>
    <execution>
      <goals>
        <goal>install</goal>
        <goal>exec</goal>
      </goals>
    </execution>
  </executions>
  <configuration>
    <version>1.9.2</version>
    <commandLineOptions>--load-images=false</commandLineOptions>
    <script>src/main/javascript/example.js</script>
    <arguments>
      <argument>arg1</argument>
      <argument>arg2</argument>
    </arguments>
  </configuration>
</plugin>
```

We should probably also support a parameter for specifying a config file instead of using the `commandLineOptions` parameter:

```xml
<plugin>
  <groupId>com.github.klieber</groupId>
  <artifactId>phantomjs-maven-plugin</artifactId>
  <version>${phantomjs-maven-plugin.version}</version>
  <executions>
    <execution>
      <goals>
        <goal>install</goal>
        <goal>exec</goal>
      </goals>
    </execution>
  </executions>
  <configuration>
    <version>1.9.2</version>
    <configFile>src/main/config/config.js</configFile>
    <script>src/main/javascript/example.js</script>
    <arguments>
      <argument>arg1</argument>
      <argument>arg2</argument>
    </arguments>
  </configuration>
</plugin>
```
For reference, here is the documentation on the [phantomjs command line options](https://github.com/ariya/phantomjs/wiki/API-Reference#command-line-options).

Will probably also want to review the parameters in the exec goal of the [exec-maven-plugin](http://mojo.codehaus.org/exec-maven-plugin/exec-mojo.html) for some other possible parameters.