Add named transparent color

[concimuscb]

Just a suggestion, not sure how many people might use it.

It would be useful to have a named color which is transparent. Currently using rgba(0, 0, 0, 0) but this is quite cumbersome.

The color could be named blank.

[MKoroteev-HORIS]

Now you can use any of the three variants for a fully transparent color:

• 'transparent' which is standard for SVG;
• 'blank' as you proposed;
• '', just an empty string for brevity, for example geom_line(color='').