ggplot: themes: element_blank() for grid

Translating element_blank() when used with grids in theme function.

Examples:
z <- z + theme(panel.grid.major =  element_blank()): https://plot.ly/~pdespouy/1489/mpg-vs-wt/
z <- z + theme(panel.grid.major.x = element_blank()): https://plot.ly/~pdespouy/1488/mpg-vs-wt/
z <- z + theme(panel.grid.major.y = element_blank()): https://plot.ly/~pdespouy/1487/mpg-vs-wt/

/cc @mkcor