Add getMainPage and setter the Creator wrapper

[jdcaballerov]

Probably upstream limitation but requiring main_page on ZimCreator creation seems weird. You allow to set it to blank but can't change it afterwards?

I used the same construct as node-libzim that fixes it from creation.
ZimCreator must implement a function getMainPage so that when finishZimCreation() (C++) is called the header is filled with it fillHeader().

With the current design I can overload the wrapper function and add a setter if needed.

I think it makes sense, yes.

[jdcaballerov]

Commit 9e45d5c Adds the feature