add the ability to highlight multiple series with setSelection

[kchengsf]

I'd like to add the ability to highlight multiple series in setSelection.

I've added a test and updated relevant documentation parts to accommodate this, however there are quite a few ways this could be implemented. Please let me know if a different approach makes more sense. In particular, repeatedly calling _renderLineChart is cleaner but requires iteration through all series each call. Moving the logic downward requires the method to understand the nuance between an array vs string option.

[coveralls]

Coverage decreased (-0.05%) to 90.006% when pulling b9b21c0 on kchengsf:master into 623dd1d on danvk:master.

[danvk]

@klausw any thoughts on this? Seems fine to me other than style guide issues.

[klausw]

I think this looks good, and it's functionality I've been wanting myself. I don't have a strong preference about the implementation. There may be useful followups such as restricting interactive highlighting to switch among the selected series, assuming that this isn't already working.