Feature/issue 196 manysliders question

Fixes #196 by adding a new question type, `manySliders` (see example definition in [feature-manysliders](https://github.com/daydreaming-experiment/parameters/blob/master/dev/feature-manysliders/grammar-v3/test.json#L12) parameters).

Basically, a page full of sliders which have all the same hints, and where the user can enter an "edit mode": there he can decide to remove some sliders, or add other ones (either predefined or user-defined). Next time the question shows up, his settings are restored.