Proxy singleton

[mfulton26]

if FullStoryExcluder instances are repeatedly created some memory and/or performance issue could arise

React.createElement is wrapped with a Proxy and unwrapping it isn't feasible (some other code may have wrapped it too so restoring the previous value doesn't work and a revocable proxy is no longer usable)

instead of creating a unique proxy instance per FullStoryExcluder instance it might be better to have a singleton proxy used by each FullStoryExcluder instance

Note: the README makes it clear that FullStoryExcluder is meant to be initialized at the root of the app but some users might want to manage separate lists of exclusion strings and do so through nested and/or multiple FullStoryExcluder instances