Add property childrenTitlesPrefixed

[pbaris]

Add property so user has the option not to prefix the children titles with the parent title.
In plugin configuration has to add

<plugin>
    ....
    <configuration>
        ...
        <childrenTitlesPrefixed>false</childrenTitlesPrefixed>
    </configuration>
</plugin>

[bsorrentino]

Hi

Thanks for contribution

Your PR increases the flexibility of plugin though, since the confluence page's title must be unique this could be dangerous