Updates

added functions for transforming PhyloPic silhouettes (flipping and rotating)
save_phylopic bg argument updated to be "transparent" by default
added geom_phylopic (#25)
vectorized add_phylopic and add_phylopic_base (#42)
recolor_phylopic now removes white backgrounds by default
fixed the handling of alpha values
get_phylopic can now return any size raster image (#50)
removed the "thumbnail" and "twitter" format options for get_phylopic
fixed how silhouettes are gathered from PhyloPic (#51)
pick_phylopic updated to allow visualization of multiple silhouettes at once (#43)
fixed add_phylopic_base for multi-panel figures
added three vignettes (#49, #55)

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