Allow SQLite auto migrations

Discussed in #624

Over time SQLite's support for DDL statements has been improving.

We currently raise sys.exit if a user tries creating an auto migration for SQLite. If you're using a recent version of SQLite, and have simple use cases (e.g. adding tables), then auto migrations will work fine.

piccolo/piccolo/apps/migrations/commands/new.py

Lines 234 to 235 in bd8ec50

    
           if auto and isinstance(engine, SQLiteEngine): 
        
               sys.exit("Auto migrations aren't currently supported by SQLite.")

So instead of raising sys.exit we will output a warning instead.

	if auto and isinstance(engine, SQLiteEngine):
	sys.exit("Auto migrations aren't currently supported by SQLite.")