Adds support to -processorpath

[ulyssesrr]

Libraries like Google AutoValue and MapStruct have split artifacts for the annotations and the actual processor. This approach combined with the "-processorpath" argument avoids pulling unnecessary classes into the runtime classpath, which I believe should be the preferred approach when possible.

This change is JDK8 compatible, as I'm hoping to use it with that version and was mostly adapted from maven-compiler-plugin.

Sample usage:

...
<properties>
    <mapstruct.version>1.4.2.Final</mapstruct.version>
</properties>
...
<dependencies>
    <dependency>
        <groupId>org.mapstruct</groupId>
        <artifactId>mapstruct</artifactId>
        <version>${mapstruct.version}</version>
    </dependency>
</dependencies>
...
<build>
    <plugins>
	<plugin>
		<groupId>org.bsc.maven</groupId>
		<artifactId>maven-processor-plugin</artifactId>
		<version>5.0-jdk8-rc1</version>
		<executions>
			<execution>
				<id>process</id>
				<goals>
					<goal>process</goal>
				</goals>
				<phase>generate-sources</phase>
			</execution>
		</executions>
		<configuration>
			<annotationProcessorPaths>
				<annotationProcessorPath>
					<groupId>org.mapstruct</groupId>
					<artifactId>mapstruct-processor</artifactId>
					<version>${mapstruct.version}</version>
				</annotationProcessorPath>
			</annotationProcessorPaths>
		</configuration>
	</plugin>
    </plugins>
</build>

Thanks for your time.

[bsorrentino]

HI @ulyssesrr thanks for contribution

This seems perfectly fit the purpose of this plugin

I'll merge your PR and arrange a new version (5.0) asap