Fix for matplotlib installation.

Moved .mplstyle files inside vis.
setup.cfg: added options.packages.find
    removed 3Dplot requirements (for now).
setup.py: moved stylesheet installation to vis.tools, now optional.
vis.tools: also added a use_style contextmanager that dynamically uses
    local styles (in package) or styles installed in user config folder.
colors/decorators/plot2d: now use the new use_style contextmanager.