Make it easier to change animation button label

[cpsievert]

This is currently quite difficult and actually best done via plotly_build() since this

plot_ly(mtcars, x = ~mpg, y = ~wt, frame = ~vs) %>% 
  animation_button(buttons = list(list(label = "Custom")))

overwrites the auto-generated buttons object. At least for users of the frame API, it probably only ever makes sense to have write access to button.label anyway, so I'm thinking we should add an explicit label argument to animation_button():

plot_ly(mtcars, x = ~mpg, y = ~wt, frame = ~vs) %>% 
  animation_button(label = "Custom", bgcolor = "red", font = list(color = "white"))