_.sortBy option for sorting null values to end

[jvincilione]

Currently, _.sortBy doesn't sort null values. I believe an optional param to decide what to do with null and possibly undefined (which currently defaults to end) would be a good feature.

It's a pretty simple addition, and I have already coded the feature into a project I'm working on, and can a do a PR if you believe this is a feature worth adding.

[megawac]

Hey, there are a number of issues currently open related to sort order for spurius values (namely NaN and undefined) #1768 #2041 #2042 and others.

We're probably going to leave null as is to avoid a breaking change; but it should be easy enough to handle in the comparator function