Allow user to provide a custom loading indicator

[MartaGlanc]

This PR adds a loader prop to MentionsInput component. This allows the user to provide their own React element to use as a loading indicator.

I initially wrote this as a patch for one of my projects at work, where the design required me to use Material UI's CircularProgress component for all loaders.

Also added a demo and documented the isLoading prop on Mention.

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.87%. Comparing base (ad1a281) to head (26f6f3c).
Report is 101 commits behind head on master.

Files with missing lines	Patch %	Lines
src/SuggestionsOverlay.js	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #414   +/-   ##
=======================================
  Coverage   78.87%   78.87%           
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  Files          31       31           
  Lines         691      691           
  Branches      107      107           
=======================================
  Hits          545      545           
  Misses        145      145           
  Partials        1        1           

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[gabrielmicko]

It would help a lot.

[gabrielmicko]

OMG, I have been thinking about this all day and it is so simple:

 customSuggestionsContainer={
                isLoading
                  ? () => <div>loading</div>
                  : children => <ul>{children}</ul>
              }