Add a parameter for the cupertino button text style

[Senne021]

Because the text color of a CupertinoDialogAction can't be change by the theme in the stable flutter version, I've added a parameter to set the color separately.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02 :tada:

Comparison is base (1238f62) 93.07% compared to head (b059976) 93.09%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #242 +/- ## ========================================== + Coverage 93.07% 93.09% +0.02% ========================================== Files 10 10 Lines 967 970 +3 ========================================== + Hits 900 903 +3 Misses 67 67 ``` | [Impacted Files](https://codecov.io/gh/larryaasen/upgrader/pull/242?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Larry+Aasen) | Coverage Δ | | |---|---|---| | [lib/src/upgrader.dart](https://codecov.io/gh/larryaasen/upgrader/pull/242?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Larry+Aasen#diff-bGliL3NyYy91cGdyYWRlci5kYXJ0) | `90.27% <100.00%> (+0.08%)` | :arrow_up: | 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=Larry+Aasen). 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=Larry+Aasen)

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[larryaasen]

@daniel-possienke Thanks for this PR. I like this new feature and would like to merge this.

However, there are a few additional changes I would like. Can you add a line to the widget test test UpgradeWidget Cupertino using cupertinoButtonTextStyle, and add the cupertinoButtonTextStyle parameter to the Customization section (alphabetized) in the README file?

[Senne021]

@larryaasen Thanks for your feedback. I've added the cupertinoButtonTextStyle to the test UpgradeWidget Cupertino and I've added the new parameter to the README file.

[larryaasen]

@daniel-possienke These latest changes look good so I will merge now. I will put out an update to the package soon. Thanks.