Support SPLIT_PART function for Postgres

We can support SPLIT_PART function for Postgres:

  split_part ( string text, delimiter text, n integer ) → text

Splits string at occurrences of delimiter and returns the n'th field (counting from one), or when n is negative, returns the |n|'th-from-last field.

split_part('abc#def#ghi', '#', 2)   # returns 'def'
split_part('abc,def,ghi,jkl', ',', -2)     # returns 'ghi'