Ability to pass async function as a source property

[dhivyada]

After this change:
If there is already a function that returns a promise, such an async function can be passed to the source prop and the results can be processed.
E.g.
<autocomplete source="async function(input) { return getResults(input) }"> </autocomplete>
Here, getResults is a function when returns a promise

[coveralls]

Pull Request Test Coverage Report for Build 71

• 11 of 31 (35.48%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-9.5%) to 86.923%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/components/Autocomplete.vue	11	31	35.48%

Totals
Change from base Build 57:	-9.5%
Covered Lines:	166
Relevant Lines:	189

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💛 - Coveralls