Merge together input data frame with the output of `fit_transform`

[seaaan]

So you could have the input data frame with two additional columns so the user doesn't need to merge them manually.

However, user will have to supply only numeric columns to the umap function so any categorical labels will be gone. This could possibly be solved by allowing the user to pass in a data frame containing all of their data and then the numeric indices of the columns to subset to. So e.g. you would call umap(iris, 1:4, ...) and then we'd run fit_transform(iris[ , 1:4]) and combine that result with iris not iris[,1:4]. This seems the most useful to me.