(api) function names: pct_change, diff
Hi, first of all thanks for the amazing library everyone!

This is a minor issue I'm sure, but I find it impossible to remember the name of the `.pct_change()` function.

Partly this is because of confusion with the existing .diff() function which is easy to remember - I always try things like:
- pctdiff()
- pct_diff()
- percent_diff()

Normally after a couple of goes I admit defeat and Google it yet again :)

So this is a request for an additional function with the name `.pct_diff()` (or whatever spelling is preferred) that replicates/calls `.pct_change()`, in order to standardise the API.

Thanks!