Add feature transformation and truncation to SKLL

[desilinguist]

It would be nice to have some more feature pre-processing functionality in SKLL, e.g., feature truncation and transformation sort of like what's in RSMTool. We could have a separate preprocessing module in SKLL that could contain these and then expose them in the main config somehow. We need to think about this a bit more.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions.

[desilinguist]

Keep it open please.

[desilinguist]

I think we have enough to do in v2.5 so I am going to move this out for a later release.