Lunar linecol gradient

[mschilli87]

This PR adds new parameters that allow coloring of the moon phase outlines by a gradient indicating the moon's progression:

# devtools::install_github("mschilli87/calendR@lunar-linecol-gradient")
library(calendR)

# Default remains unchanged.
calendR(year = 2021, month = 12, lunar = TRUE)

# Default gradient colors phases smoothly in a circular fashion:
calendR(year = 2021, month = 12, lunar = TRUE, lunar.line.gradient = TRUE)

# Parameters allow complex custom coloring:
calendR(year = 2021, month = 12, lunar = TRUE, lunar.line.gradient = TRUE,
        low.col = "yellow", lunar.col = "orange",
        lunar.linecol.new = "black", lunar.linecol.waxing = "purple",
        lunar.linecol.full = "blue", lunar.linecol.waning = "cyan",
        lunar.linecol.old = "green")

Note that roxygen2::roxigenise picked up some unrelated documentation changes I commited these separately for clarity.

Please let me know if you prefer any changes to my suggestion.