Add phylopic_key_glyph

[willgearty]

This adds a custom key glyph function that can be used with geom_phylopic to specify silhouettes for the legend.

Resolves #57.

[willgearty]

Note: I still need to figure out why one of the tests isn't making the correct snapshot.

[willgearty]

With the most recent commit, looks like this "just works" with the new fill/color split.

[willgearty]

This is the expected behavior for a ggplot legend (which should only represent the aesthetics). For example:

ggplot(df) +
    geom_point(aes(x = x, y = y, fill = name), color = c("blue", "red"), size = 10, shape = 21) +
    coord_cartesian(xlim = c(1,6), ylim = c(5, 30))

[willgearty]

OK, your two other requests/comments have been moved to separate issues for future development. I'll merge this once the checks pass!