winsorize factors

[mattansb]

Originally posted #47

Current behavior: factors aren't winsorized.

Suggested behavior: Add include_factors (defaults to FALSE for backward comp) to allow some for of winsorizing, where levels with low frequency -- defined by threshold -- to be combined to ".other".

[IndrajeetPatil]

Sounds good to me. Wanna work on this?

[mattansb]

Sure.