Release 0.6

New stuff:

1. Support for overlapping features.
2. Support for using floium.Map objects as base maps.
3. Sexier colormaps.
4. New parameter strip_na for Autotyp and Phoible.
5. Changed default colors for colormaps and gradients to 'from white to green'.
6. Added citation attribute for lingtypology.db_apis.Wals (citation for given features).
7. Support for all kind of shit types that can be processed while drawing a colormap. Now you can give: a list/tuple/pandas.Series of int, str (converted to int or float), float (converted to int if they are integer).