Dynamically infer varchar column name

[Arp-G]

We must dynamically infer the column's varchar size by finding the max data size in that column. So the varchar_limit the user specifies must only be used to decide when to switch from varchar to text.

Helps in fixing: https://github.com/Arp-G/csv2sql/issues/25

Also contains some minor cleanup and fixes.

[Arp-G]

for empty columns set varchar(1)