make StartMojo thread-safe.

Hi, thanks for your great plugin. It is very useful for me and my team.

Today I have a suggestion to make your plugin thread-safe. This change lets user uses `-T` option of `mvn` command. Here is [a document](https://cwiki.apache.org/confluence/display/MAVEN/Parallel+builds+in+Maven+3#ParallelbuildsinMaven3-Mojothreadsafetyassertionchecklist) which explains requirement to make mojo thread-safe.

In this change there are 2 key points:

* add `threadSafe` parameter to `@Mojo` annotation.
* removing `AnsiConsole.systemInstall()`.
    * It affects `System.out` and `System.err` so other threads will be affected. But this mojo doesn't call `System.out` and `System.err` directly, so I expect that we can remove this method calling.

Thanks in advance,
Kengo TODA