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.

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/signavio/react-mentions/mll3l1eki
✅ Preview: https://react-mentions-git-fork-martaglanc-feature-custom-loader-a9ede6.signavio.vercel.app

[codecov-commenter]

Codecov Report

Merging #414 into master will not change coverage.
The diff coverage is 0.00%.

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

Impacted Files	Coverage Δ
src/MentionsInput.js	68.75% <ø> (ø)
src/SuggestionsOverlay.js	66.00% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ad1a281...26f6f3c. Read the comment docs.

[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>
              }