Update main.py
Update main.py

- Now user may specify the type of database they want in command line
  Usage: python3 main.py db_type_option
  Where db_type_option can be:
    btree: "btree" or "-btree" or "-b"
    hash: "hash" or "-hash" or "-h"
    indexfile: "indexfile" or "-indexfile" or "-i"
- Now if user does not create database in the first step, the program shows a warning and asks user if they wish to continue.
  If they do, the program may crash, or give unexpected result
- So far the program does not drop the database before quit the program
nurnbeck/project-2-CMPUT-291