allow for multiple glyphs per plot

[leeper]

This PR enables multiple glyphs on the same plot by changing how use_glyph is passed through rep(), for example:

# one glyph per group (mapped to order of `parts`)
waffle::waffle(c(5,5,5,5), rows = 2, use_glyph = c("male", "female", "male", "female"))

# one glyph per "square"
waffle::waffle(c(5,5,5,5), rows = 2, use_glyph = rep(c("male", "female"), 10))