Added option to make indexes paths more pretty.

Hi,
this PR implements parts of the feature described in #1548. If ```PRETTY_URLS``` and ```INDEXES_PRETTY_PAGE_URL``` are both set to ```True```, index pages get not named ```index-n.html```, but ```page/m/index.html```.

Missing: in case ```INDEXES_PAGES_MAIN``` is set to ```True```, redirects for ```page/1/index.html``` (resp. ```page/n/index.html```, depending on ```INDEXES_STATIC``` config) should be created to the main index page.

Cheers,
Felix

fixes #1548.
While we are at this: should it be possible to configure that depending on the language? So you can have URLs like ```page/1/``` for english, ```seite/1/``` for german, ...? That would require also passing the language to ```adjust_name_for_index_path_list```, i.e. adding a new parameter. (In my opinion, it's not necessary to configure that per language (in the same site), and one setting per site is enough.)