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

The ```HtmlFile``` class has several properties that permit to change several parts 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:
* ```showTab``` for enabling the generation of the tabs "Spec" and "Source"
* ```showSource``` for enabling the generation of the tab "Source", specifically.
* ```specTabLabel``` for changing the label of the "Spec" tab.
* ```sourceTabLabel``` for changing the label of the "Source" tab.
* ```footer``` for changing the text generated at the foot of the pages.