Added option to disable Dialog action buttons. Updated Dialog docs.

[ShayanJavadi]

I added the option to disable Dialog buttons. This is specially useful for handling "Affirmative Actions" as noted here in the Material Design guidelines (scroll down to where it says "Languages without capitalization" and you can see what I'm referring to under the picture with Korean in it)

So for example if you had an action called "cancel" and wanted to disable it you would do:

<DialogDefaultActions
   actions={['cancel', 'ok']}
   /**
   * this will disable the button for "cancel"
   */
   options={{ cancel: { disabled: true } }}
   onActionPress={() => {}}
/>

I also updated the docs for Dialog to reflect the changes.

My changes are reflected in both the stacked and default actions.

Let me know if you have any comments or questions!

here's screenshots of it in action:

[codecov-io]

Codecov Report

Merging #316 into master will not change coverage.
The diff coverage is n/a.

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##           master     #316   +/-   ##
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  Coverage   76.64%   76.64%           
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  Files          19       19           
  Lines         775      775           
  Branches      114      114           
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  Hits          594      594           
  Misses        150      150           
  Partials       31       31

[ShayanJavadi]

Hey @xotahal did you get a chance to review this pr yet?

[janziemba]

Approved. Thank you.