Allow root file to be excluded and set filename of hierarchy files

[svenevs]

I have some questions about this.
For our project this is a breaking change.
We used those files directly in a toctree. Which is not possible anymore due to the .include suffix. Is it still possible to do this?

Originally posted by @IceflowRE in #136 (comment)

Interesting. Until it was brought up I wasn't aware of or intending the original behavior. IIRC it means each directive gets processed twice just one page isn't linked to or something so the default of .include is going to be kept.

But it's easy to patch in a config value for exhale_args to just allow you to choose the file basename (string) so that you can restore the original behavior. Would that work?

[IceflowRE]

Of course that would work. Or just an option that wont add the .include suffix and do not generate the rootFileName file?