Option to ignore undefined columns

[kenhosr]

Git commit version: 3be31fb

Currently, in the schema, we need to define every column, even when some columns we actually don't care, we still need to list them and associate empty rule with them.

With a global directive to ignore undefined columns, means apply no rule to the un-defined columns, it will be helpful.

[DavidUnderdown]

We'll give this some thought - it would have to be an option mutually exclusive to @totalColumns as that verifies that the number of rules matches the number of columns in the file being validated.