Refactor visualizations and add plotly support

• Enable plotting periodic tables with plotly as well as bokeh backends
  through mendeleev.vis.plotly.periodic_table_plotly and mendeleev.vis.bokeh.periodic_table_bokeh
  functions.
• Add mendeleev.vis.periodic_table function for convenient periodic table plotting wrapping both plotting
  backends.
• Refactored the mendeleev.vis module so it can be wasily extended with plotting backends.
• Add CITATION.cff file.