Add loading spinner props to react-native-elements ButtonProps

[awinograd]

• Use a meaningful title for the pull request. Include the name of the package modified.
• Test the change in your own code. (Compile and run.)
• Follow the advice from the readme.
• Avoid common mistakes.
• Run npm run lint package-name (or tsc if no tslint.json is present).

If changing an existing definition:

• Provide a URL to documentation or source code which provides context for the suggested changes: https://react-native-training.github.io/react-native-elements/API/buttons/
• Increase the version number in the header if appropriate. n/a
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[dt-bot]

types/react-native-elements/index.d.ts

to authors (@iRoachie @ifiokjr). Could you review this PR?
👍 or 👎?

[RyanCavanaugh]

@awinograd Please fix the failures indicated in the Travis CI log.

[awinograd]

Hi @RyanCavanaugh, I'm not sure how I could have caused the jenkins failure. Can you point me in the right direction?

Error: mock-knex depends on knex but has a lower required TypeScript version.

I get that locally running npm test on master as well

[ghost]

Hi @awinograd, that was fixed in a different PR (#21384). See updated failure messages -- just a few lint errors.

[awinograd]

@andy-ms i've rebased on master (to get the fix in #21384) and have fixed the lint errors

[typescript-bot]

@awinograd Please address the merge conflict.

[awinograd]

Merge conflict fixed. Ready for re-review