Added environment variables management functionalities to plugin

HI, 
I needed to inject some environment variables during the execution of npm process due to proxy company settings.

So I added the possibility to configure the plugin in mvn with following tags:

```xml
....
<goals>
  <goal>npm</goal>
</goals>
<configuration>
  <environmentVariables>
    <property>
     <name>NODE_TLS_REJECT_UNAUTHORIZED</name>
     <value>0</value>
    </property>
  </environmentVariables>
</configuration>
</execution>
....
```

With default values if not defined.

Cheers
G.