Too many values for palette

When more than 8 quantiles are requested, we get an error from too few unique color values.

> Warning: This manual palette can handle a maximum of 8 values. You have supplied

Minimum reproducible example

```
library(quantileplot)
library(tidyverse)
d <- data.frame(x = rnorm(100)) %>%
  mutate(y = rnorm(n(),x^2))
quantileplot(y ~ x, data = d, quantiles = seq(.1,.9,.1))
```