Make filter container indexSearch prop optional in types

[connorholyday]

The new indexSearch prop introduced for FilteringContainer in #123 is great and has a default so if I'm not mistaken it should be listed as optional in the types.

My TypeScript is throwing a tantrum at the moment because it's expecting me to pass a custom function for this

Thanks for all the work on this package!

[coveralls]

Coverage remained the same at 94.498% when pulling 24b5cfc099089ccc8d218f82bdd94c911b35676d on connorholyday:patch-1 into dfaaf7402d56a3eaf1034fbebfc86770d41f3700 on diogofcunha:master.

[jeremypeters]

@diogofcunha It would be great if this could be merged :)