String find and replace

[sharplet]

I think it would be worth supporting some find and replace functions with support for back-references. Here's the API I have in mind:

extension String {

  /// Replace the first occurrence of `pattern` with `replacement`.
  func stringByReplacingFirstMatching(pattern: String, with replacement: String) -> String

  /// Replace all occurrences of `pattern` with `replacement`.
  func stringByReplacingAllMatching(pattern: String, with replacement: String) -> String

  /// Mutating form of `stringByReplacingFirstMatching(_:with:)`.
  mutating func replaceFirstMatching(pattern: String, with replacement: String)

  /// Mutating form of `stringByReplacingAllMatching(_:with:)`.
  mutating func replaceAllMatching(pattern: String, with replacement: String)

}

We should be able to implement this with NSRegularExpression.stringByReplacingMatchesInString(_:options:range:withTemplate:) and NSRegularExpression.replaceMatchesInString(_:options:range:withTemplate:).