ggplot: themes: Font styles and Bold

From Asana task: https://app.asana.com/0/15417658028776/15417658028776

- Add family fonts to title, axis title, legend and text. To use like in: p <- p + theme(plot.title=element_text(family="Raleway"))
- Add bold, italic and bold.italic face to title, axis title and text. To use like in: p <- p + theme(plot.title=element_text(face="bold"))

Comments:
Family font for axis is already integrated.
Legend Title is not implemented in Plotly yet. 
Legend and ticks elements are hidden in the JSON. Not evident to concatenate HTML elements to it. 

Examples:
Title, face=“bold”: https://plot.ly/~pdespouy/1390/scatter-plot-example/
Title, family=“Times”: https://plot.ly/~pdespouy/1389/scatter-plot-example/
Title, face=“bold” and family=“Times”: https://plot.ly/~pdespouy/1391/scatter-plot-example/
xaxis, face=“bold” and family=“Raleway”; yaxis, family=“Raleway”: https://plot.ly/~pdespouy/1392/scatter-plot-example/
Legend, family=“Raleway”: https://plot.ly/~pdespouy/1393/scatter-plot-example/
Text, family=“Raleway”: https://plot.ly/~pdespouy/1394/scatter-plot-example/
Text, face=“italic”: https://plot.ly/~pdespouy/1401/scatter-plot-example/
Text, face=“bold.italic”: https://plot.ly/~pdespouy/1402/scatter-plot-example/
Text, family=“Raleway”: face=bold.italic: https://plot.ly/~pdespouy/1403/scatter-plot-example/

cc// @mkcor 
By the way, let's improve this:
`if (length(trace.list) < 2)`