Add keepMounted option and fix screen glitch

[pikaju]

Adds option that keeps the Modal mounted even when it is hidden. This is useful for keeping the DOM state inside the Modal as well as for SEO purposes.

Fixes #233 and #495

[pradel]

Thanks for the pr! Could you please add an example in the docs so I can try it?

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.16 :tada:

Comparison is base (16aad65) 95.28% compared to head (d2c9bb4) 95.45%.

:exclamation: Current head d2c9bb4 differs from pull request most recent head 11fd42a. Consider uploading reports for the commit 11fd42a to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #516 +/- ## ========================================== + Coverage 95.28% 95.45% +0.16% ========================================== Files 6 6 Lines 191 198 +7 Branches 69 74 +5 ========================================== + Hits 182 189 +7 Misses 9 9 ``` | [Impacted Files](https://codecov.io/gh/pradel/react-responsive-modal/pull/516?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=L%C3%A9o+Pradel) | Coverage Δ | | |---|---|---| | [react-responsive-modal/src/index.tsx](https://codecov.io/gh/pradel/react-responsive-modal/pull/516?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=L%C3%A9o+Pradel#diff-cmVhY3QtcmVzcG9uc2l2ZS1tb2RhbC9zcmMvaW5kZXgudHN4) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=L%C3%A9o+Pradel). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=L%C3%A9o+Pradel)

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[pikaju]

Hey @pradel, thanks for the response. I added an example and docs in the Next project.

I also changed how modals are centered, from the "inline" behavior to flex-box centering. I did this because there was an issue when the modal had little space (e.g. on a mobile device) and the pseudo-element you had before was wrapped to a new line. This implementation doesn't do that.

[pradel]

@pikaju could you please split the pr into two parts?

• the keepMounted can be released as a minor version first
• then the change you made to the CSS for the mobile issue, this one will need to be released as a breaking change as it may conflict with the CSS used by users of this package

[pikaju]

@pradel Sorry, but that doesn't seem worth the effort, I suggest just releasing a new major version