Added unpackOutputDirectory and includeNonClassFiles to the unpack goal

[lyubomyr-shaydariv]

This PR adds new two configuration options to make the unpack goal more precise:

• unpackOutputDirectory to change the originally hardcoded android-classes directory.
• includeNonClassFilesto allow non-.class files that are not specified in the manifest.

Rationale

Since I have to use Retrolambda, I have to extract v52-bytecode JAR classes from the repository before downgrading the classes bytecode. As of 2018-09-26, I used the following methods described here:

• Use org.apache.maven.plugins:maven-dependency-plugin to extract non-AAR dependencies.
• Use org.apache.maven.plugins:maven-antrun-plugin to extract AAR contents because org.apache.maven.plugins:maven-dependency-plugin can't do the job.
• Use net.orfjackal.retrolambda:retrolambda-maven-plugin to downgrade the bytecode.
• Use com.simpligility.maven.plugins:android-maven-plugin with the exclusion filter enabled to process downgraded stuff only.

Since the use of org.apache.maven.plugins:maven-dependency-plugin and org.apache.maven.plugins:maven-antrun-plugin is error-prone (concrete groups IDs, artifact IDs and versions should be specified directly in paths), it turns out that the unpack goal from net.orfjackal.retrolambda:retrolambda-maven-plugin can replace the first two plugins. For example,

<plugin>
	<groupId>com.simpligility.maven.plugins</groupId>
	<artifactId>android-maven-plugin</artifactId>
	<executions>
		<execution>
			<phase>process-classes</phase>
			<goals>
				<goal>unpack</goal>
			</goals>
			<!--
			Configuration inherited.
			-->
			<configuration>
				<!--
				unpackOutputDirectory is useful to keep all classes
				in a single directory in order to use the Retrolambda plugin
				that seems to support one input directory only
				-->
				<unpackOutputDirectory>${project.build.directory}/classes</unpackOutputDirectory>
				<!--
				includeNonClassFiles is useful to extract all JAR entries,
				even those that are not included to the META-INF registry.
				For example, the Flyway library Android implementation works
				with the migrations int the assets, but still uses some content
				like version.txt that's provided as a part of the dependency JAR
				as a regular Java resource file.
				-->
				<includeNonClassFiles>true</includeNonClassFiles>
			</configuration>
		</execution>
	</executions>
	<configuration>
		<!--
		We specified the source entries to extract previously.
		-->
		<skipDependencies>true</skipDependencies>
	</configuration>
</plugin>

<plugin>
	<groupId>net.orfjackal.retrolambda</groupId>
	<artifactId>retrolambda-maven-plugin</artifactId>
	<executions>
		<execution>
			<phase>process-classes</phase>
			<goals>
				<goal>process-main</goal>
			</goals>
		</execution>
	</executions>
</plugin>

<!--
This step is not even necessary now -- it's default and does not require additional configuration.
Please note that the configuration section with the exclusion can be omitted as well.
<plugin>
	<groupId>com.simpligility.maven.plugins</groupId>
	<artifactId>android-maven-plugin</artifactId>
</plugin>
-->