Change the content of the generated HTML files from the Maven plugin

With this pull request, the ```HtmlFile``` class exhibits properties that permit to change the content of the generated files. Some of these properties may be set from the Maven's configuration of ```jnario-maven-report-plugin```.

The added properties are:
* ```showTabs``` for enabling the generation of the tabs "Spec" and "Source" (also configurable from Maven).
* ```showSource``` for enabling the generation of the tab "Source", specifically.
* ```specTabLabel``` for changing the label of the "Spec" tab  (also configurable from Maven).
* ```sourceTabLabel``` for changing the label of the "Source" tab (also configurable from Maven).
* ```footer``` for changing the text generated at the foot of the pages (also configurable from Maven).