Allow non-array values to be added to Crossfilter

[Wildhoney]

If you pass an object to the add method then it will fail silently. Crossfilter is expecting an array. There are a few solutions that I thought of:

• Notify the developer if they attempt to add an object;
• Change the name of the add method to something that implies pluralisation, such as addRecords/addMany;
• Transform the value into an array if it isn't already;

I chose the last one, but happy to discuss alternatives.

[jkleint]

+1; just got bitten by the same problem. Thanks.

[RandomEtc]

Thanks for your contributions and sorry for silence on this side. As discussed in #151 an active fork is being developed in a new Crossfilter Organization. Please consider rebasing and opening your PR there (if you haven't already) where it should be warmly welcomed by the new maintainers. Cheers!