[maven plugin] Use configurator for maven plugin

[jimschubert]

see #2104

This reduces code duplication between CLI and maven-plugin.
The maven plugin can now support more options like language-specific-primitives and additional-properties.

Here's an example configuration to demonstrate passing additional-properties:

<plugin>
      <groupId>io.swagger</groupId>
      <artifactId>swagger-codegen-maven-plugin</artifactId>
      <version>2.1.6-SNAPSHOT</version>
      <executions>
          <execution>
              <goals>
                  <goal>generate</goal>
              </goals>
              <configuration>
                  <inputSpec>/Users/jim/projects/swagger-codegen/modules/swagger-codegen-maven-plugin/examples/swagger.yaml</inputSpec>
                  <language>csharp</language>
                  <configOptions>
                      <additional-properties>optionalProjectFile=true</additional-properties>
                  </configOptions>
              </configuration>
          </execution>
      </executions>
  </plugin>

[ePaul]

                  <configOptions>
                      <additional-properties>optionalProjectFile=true</additional-properties>
                  </configOptions>

I guess this is out of scope for this pull request, but I think that additional-properties should be of type Map, not string, and thus allow structured entry:

                  <configOptions>
                      <additional-properties>
                          <optionalProjectFile>true</optionalProjectFile>
                      </additional-properties>
                  </configOptions>

I'm not sure how one could do a transition supporting both old and new form, though.

[ePaul]

"to facilitate testing"?

[jimschubert]

Thanks. Must've been muscle memory from the day job.