Exploring new geoms for diverging plots

[larmarange]

• renaming position_likert_count() into position_diverging()
• add an option default_by in stat_ptop()
• geom_bar_diverging(), geom_bar_likert() and geom_bar_pyramid()
• geom_text_diverging(), geom_text_likert() and geom_text_pyramid()
• geom_bar_prop() and geom_text_prop()
• improve/clean doc
• add vjsut as a parameter to geom_*_text()
• add some tests
• add a vignette