Fix logticks color (#767)

This fixes #767. I adapted some of the code from `annotate`. I'm not sure this is the cleanest way to do it, but it works.

Examples:

```r
df <- data.frame(x=1, y=c(1,10,100,1000), z=c('a','b','a','b'))
p <- ggplot(df, aes(x=x, y=y, colour=z))+
  geom_point() +
  scale_y_log10() +
  scale_color_manual(values=c("red", "black"))

p + annotation_logticks(sides="lrtb")

p + annotation_logticks(sides="lrtb", alpha = .3, colour = "red")

p + annotation_logticks(sides="lrtb", alpha = .3, color = "red")

```