Support interactive use

It seems that the plugin has some support for running interactive programs. I am trying to run activator from play framework when I enable a specific profile. This seems to work, except that the prompts and the text I enter do not appear until after I press return.

This may be because the input and output is being buffered and beyond the ability of the plugin to fix.

I was able to get a similar effect with this:

               <plugin>
                    <groupId>org.codehaus.mojo</groupId>
                    <artifactId>exec-maven-plugin</artifactId>
                    <executions>
                        <execution>
                            <id>runActivator</id>
                            <phase>integration-test</phase>
                            <goals>
                                <goal>exec</goal>
                            </goals>
                            <configuration>
                                <executable>bash</executable>
                            </configuration>
                        </execution>
                    </executions>
                </plugin>

bash is the standard Linux shell, although I was running it on a Mac.

Start by reproducing the reported behavior by running Activator through the plugin with the shown profile configuration on macOS or Linux. Inspect how the plugin connects child-process input and output, and compare the behavior with the referenced exec-maven-plugin setup. Done means interactive prompts and typed text appear without waiting for Return.