[R-Forge #4958] sep = "" option for fread

Submitted by: Eduard Antonyan; Assigned to: Nobody; [R-Forge link](https://r-forge.r-project.org/tracker/index.php?func=detail&aid=4958&group_id=240&atid=978)

The meaning would be the same as for read.table and would read several whitespace characters in a row as a single separator, which is a pretty common way of presenting data.
