Add overload of createIndex that takes an array instead of variadic parameters

[bbrk24]

Currently, the Table.createIndex method only has one overload:

public func createIndex(_ columns: Expressible..., unique: Bool = false, ifNotExists: Bool = false) -> String

It would be nice to have a second overload like

public func createIndex(_ columns: [Expressible], unique: Bool = false, ifNotExists: Bool = false) -> String

so that it becomes possible to wrap it in a helper function that can take an arbitrary number of columns in its parameters. Currently, there's no way to forward variadic arguments.

[nathanfallet]

Fixed by #1326