Prop source cannot be a computed/method that returns array

[carsonreinke]

Right now, providing a function for prop source requires that it returns a string. This should be changed to check what the function returns, as this may return an array instead of a string.

There is then no way to use a computed or method for the source prop, unless it returns a string.

https://github.com/charliekassel/vuejs-autocomplete/blob/master/src/components/Autocomplete.vue#L244

https://github.com/charliekassel/vuejs-autocomplete/blob/master/src/components/Autocomplete.vue#L250

I'd be happy to put in a PR if this is agreeable.