Add skip flags

[korthout]

This introduces 3 flags:

• flatten.skip: skips the entire plugin regardless of goal
• flatten.clean.skip: skips the clean goal
• flatten.flatten.skip: skips the flatten goal

This means the user has control over what parts are skipped, and the flags are descriptive. The flatten. prefix is in line with other parameters like flatten.mode and flatten.dependency.mode.

Test cases have been added for all three flags. Of course, I also tested it locally. The resulting logs are as follows:

[INFO] --- flatten:1.5.1-SNAPSHOT:clean (flatten.clean) @ my-test-module ---
[INFO] Clean skipped.

[INFO] --- flatten:1.5.1-SNAPSHOT:flatten (flatten) @ my-test-module ---
[INFO] Flatten skipped.

Usage is either through passing parameters along with the command:

mvn <goal> -Dflatten.skip
mvn <goal> -Dflatten.clean.skip
mvn <goal> -Dflatten.flatten.skip

Or through the configuration:

<configuration>
  <skip>false</skip>
  <skipClean>false</skipClean>
  <skipFlatten>true</skipFlatten>
</<configuration>

closes #290

[korthout]

@slawekjaranowski PR includes tests and is ready for review. Let me know if there's anything unclear or missing.

[slawekjaranowski]

@korthout thanks

[korthout]

@slawekjaranowski I'm curious about your release policy. Would you be willing to publish a release for this?