Add subset and transform
Also reorganize `[` & `[[` to subset instead of select

Note: for transform, transform is very similar to mutate. Spark doesn't seem to replace existing column with the name in mutate (ie. `mutate(df, age = df$age + 2)` - returned DataFrame has 2 columns with the same name 'age'), so therefore not doing that for now in transform.
Though it is clearly stated it should replace column with matching name (should I open a JIRA for mutate/transform?)

Author: felixcheung <felixcheung_m@hotmail.com>

Closes #8503 from felixcheung/rsubset_transform.