Optimization

• Ability to add multiple e_title.
• draw argument added to e_charts, to set whether to draw the chart, see bullet below.
• e_draw_p added to choose draw chart.
• e_inspect to inspect the output of echarts4r.
• echarts_from_json to create an echarts4r object from JSON options.
• e_axis family of functions (e.g.: e_x_axis) now accepts a column name as first argument to easily adapt the range of the axes, in a similar fashion as with e_visual_map.
• e_charts family forces data.frame conversion to avoid jsonlite bug.
• Fixed bug in the formatter argument of e_tooltip where passing custom function was not working.