Refactoring and unifying indices, and adding indices to archives

Hi,
this is a pull request for some refactoring and new features discussed in Issue #1537. Namely:

 * ```nikola/plugins/task/indexes.py``` and ```nikola/plugins/task/tags.py``` do no longer share similar, but not identical code to create indexes;
 * class ```Nikola``` in ```nikola/nikola.py``` contains a method ```generic_index_renderer```;
 * new option ```INDEXES_PAGES_STATIC``` to control which posts are put into which index files; the default ```True``` has the same behavior as the old ```indexes.py```, and ```False``` has the same behavior as the old ```tags.py```;
 * page numbering in caes ```INDEXES_PAGES_STATIC = True``` is more logical;
 * post lists in archives can be shown as indices as well (new config option ```ARCHIVS_ARE_INDEXES```).

Cheers,
Felix