Create index support array as parameters to solve #1322

[hawk0620]

Thanks for taking the time to submit a pull request.

Before submitting, please do the following:

• Run make lint to check if there are any format errors (install swiftlint first)
• Run swift test to see if the tests pass.
• Write new tests for new functionality.
• Update documentation comments where applicable.

overload public func createIndex(_ columns: [Expressible], unique: Bool = false, ifNotExists: Bool = false) -> String and public func dropIndex(_ columns: [Expressible], ifExists: Bool = false) -> String to solve #1322

[nathanfallet]

LGTM 👍
Thank you!