Add attributes support to Element.

Now you can use attributes:

```Java
executeMojo(
                plugin(
                        groupId("org.apache.maven.plugins"),
                        artifactId("maven-antrun-plugin"),
                        version("1.7")
                ),
                goal("run"),
                configuration(
                        element(name("target"),
                                element(name("echo"), "Execute npm install -g on ${basedir}" + workingDirectory),
                                element(name("exec"),
                                        element("arg").attribute("line", "install -g")
                                )
                                        .attribute("executable", "npm")
                                        .attribute("dir", "${basedir}" + workingDirectory)
                                        .attribute("failonerror", "true")
                        )
                ),
                executionEnvironment(
                        mavenProject,
                        mavenSession,
                        pluginManager
                )
        );
```