[Dropdown] - allowTab tabindex value should default to select's tabindex (if exists)

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Steps to Reproduce

1. Integrate Dropdown only via /dist/components/dropdown.min.js; /dist/components/dropdown.min.css
2. Initialize on existing, traditional HTML select (with options etc) with an existing tabindex attribute, using ui search selection class 'triggers' and $('element').dropdown()

Expected
Dropdown created with tabindex value copied from existing select element's tabindex value.

Result
Dropdown created with tabindex="0"

Relevant line appears to be:

Semantic-UI/src/definitions/modules/dropdown.js

Line 2266 in daa2438

.attr('tabindex', 0)

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Slapdash 'solution':

.attr('tabindex', element.attributes.tabindex.value)

No doubt for wider use requires a little more refinement. Will submit PR if/when I can :)