Adds get_input_text() that can be used to get the literal variable name and make_named_data() that creates a list with the input data frames. This second one is necessary because the inputs may be quosures and we need to get the underlying data frames. Default names are $x and $y.

animate_join() now lives in animate_join.R with the other animate_join_ functions. These all use rlang::enquo() to capture their arguments so that animate_join() can print the variable name in the plot title.