Set default value for `append_dim` as `dims` inside `apply_append`

[glennmoy]

Looking at some tests repeating dims=x, append_dim=x made me think, would it make sense to set the default value for append_dim as dims inside the function? Maybe that only works when dims is single, as is currently the case.

Originally posted by @bencottier in https://github.com/invenia/FeatureTransforms.jl/pull/69#discussion_r604254552