fread should distinguish between quoted and unquoted empty strings

[st-pasha]

See #2214

For a file like this:

A,B
1,"foo"
2,
3,""

the B column should be understood as c("foo", NA, ""), not c("foo", "", "") or c("foo", NA, NA).

The proposed mechanism is the following: make 2 "string" types. The first type (default) reads empty fields as empty strings. However this type fails to recognize field "" as valid input and instead type-bumps. The second type treats empty field as NA and "" as an empty string.