Support variant for FeatureTagHelper

[zhiyuanliang-ms]

We've already offer <feature> tags which can be used to conditionally render content based on whether a feature is enabled or not.

<feature name="FeatureX">
  <p>This can only be seen if 'FeatureX' is enabled.</p>
</feature>

We want to extend the feature tag helper to support variant.

The usage will be like:

<feature name=”Alpha” variant=”V1”> Content only for Variant 1</feature> 
<feature name=”Alpha” variant=”V2> Content only for Variant 2</feature> 

[zhenlan]

It will be helpful if you can add description and examples of usage.

[zhiyuanliang-ms]

README updated.

[zhiyuanliang-ms]

@jimmyca15 @rossgrambo Could you help review/approve this PR?