ggplot: themes: dashed and dotted grid lines

ggplot: themes: dashed and dotted grid lines as lines with alpha=0.1

- Temporary solution until `dashed` and `dotted` grid lines are implemented in Plotly.
- `toRGB()` function was modified as to accept alpha values.
- Styling was done in the `test-ggplot-theme.R` script.

Example: https://plot.ly/~pdespouy/1573/scatter-plot-example/

cc/ @mkcor 
This is the default (see docs: `col2rgb(col, alpha = FALSE)`) so I would keep it short and same as before: `rgb.matrix <- col2rgb(x)`.