skip configuration parameter created: skip plugin execution

I've added skip configuration property. It is skipping execution of plugin.
```xml
<profile>
    <id>local</id>
    <properties>
        ...
        <skip.frontend>true</skip.frontend>
        ...
    </properties>
</profile>
```
And set in the _skip_ option in the plugin configuration:
```xml
<plugin>
    <groupId>com.github.eirslett</groupId>
    <artifactId>frontend-maven-plugin</artifactId>
    <version>...</version>
    <!-- optional -->
    <configuration>
        ...
        <skip>${skip.frontend}</skip>
    </configuration>
    <executions>
      ...
    </executions>
</plugin>
```

__Background:__ I have a Spring Boot REST project with AngularJS client. If a server side developer build the project, then unneccessary build client.

__Test:__ I added a test for testing skip configuration and default behaviour.

__Refactored__: FrontendMavenMojo has been created for all Mojo with skip member. It may be contains _workingDirectory_ in the future.