persist the blur event so clients can use it

[triptate]

This fixes a bug where the event that is passed to the onBlur prop will be nullified because the code is executed asynchronously using a timeout. I expect there's a good reason why the timeout is necessary; however, if it were not necessary, then removing the timeout would be a simpler and more efficient solution to this problem since event.persist() creates a copy of the event.

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[codecov]

Codecov Report

Merging #24 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@ Coverage Diff @@## master #24 +/- ##
=======================================
Coverage 99.13% 99.13% =======================================
Files 6 6 Lines 230 232 +2 Branches 52 53 +1 =======================================
+ Hits 228 230 +2 
Misses 2 2 

Impacted Files	Coverage Δ
src/Mentions.tsx	99.30% <100.00%> (+<0.01%)	⬆️

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[triptate]

@afc163 this one didn't seem to need a test case; however, an alternative strategy would be to add a prop that allows the client to indicate if they would like the event to be persisted in this case (e.g. persistOnBlur). in this case, this code would only run conditionally if the prop is included, and it would need a test case as well. let me know if you'd like me to rewrite this using that approach.