Exposing transformations applied by Cleaner and TableVectorizer

[emassoulie]

Closes #1265
Adds a method to the Cleaner and TableVectorizer transformers enabling them to give a human-readable list of the transformations (DropUninformative, ToFloat, ToDatetime) they are set to apply once fitted.

[emassoulie]

I've got something going with the transformation lists for both transformers! I feel the TableVectorizer transformations could be equally relevant to expose by raw transformer type (ToFloat, ToDatetime etc, like the Cleaner), or by transformer category ("numeric", "low cardinality" etc, since they are customisable). I'm thinking of going towards the latter, but I wanted to ask for a second opinion before I go ahead.