Add Swift Package Manager support

This version only adds Swift Package Manager support.

Fix an iOS 12 regression

• Fixes a regression in 3.3 which caused a crash on iOS 12. Turns out that availability flag for lineBreakStrategy only works on iOS 13+!

Fix a couple bugs

• Fixes tappability of regions relocated to a new line to avoid single-word last lines by UILabel. This requires compiling using Xcode 12, but works backwards-compatibly.
• Fixes accessibility elements sometimes exposing more than 1 element per tappable span.

Fix accessibility activation in some cases

• Fixes handling accessibility element activation that spans multiple lines.

Fix accessibility elements, add accessibilityLabel override

• Adds accessibilityDelegate method to override accessibilityLabel of constituent elements.
• Fixes accessibility elements not resetting when text changed.

Improved accessibility, 3D Touch support

• Updates touch handling, improving the long-press experience and adding support for 3D Touch.
• Introduces an accessibility delegate, to provide actions for links within the label.
• Adds the ability to get attributes at a point, particularly for use with 3D Touch.
• Updated examples for Xcode 10.2 and Swift 5 (the pod itself did not need updates).
• Fixes long press delegate call not being invoked unless there was a foreground or background highlight state.
• Fixes not handling adjustsFontForContentSizeCategory. Note that you do need to specify a NSFontAttributeName attribute for this to work, even on vanilla UILabel, when you are setting attributedText.
• Now requires iOS 10 or later, specifically for -[UIAccessibilityElement accessibilityFrameInContainerSpace].

Xcode 9 support

Updated to support Xcode 9 and Swift 4.

Small bug fix

• Fixes some cases where the last line in a label, when sized to fit, would not be tappable on iOS 10.

Long-press support

• Long press support: set a longPressDelegate (like tapDelegate) and be notified when the user long-presses.

Swift and Interface Builder happiness

• Adds annotations for nullability and generics to ease Swift import.
• Fixes a couple Interface Builder issues:
  • Flag the delegate as an IBOutlet so it can be assigned.
  • Fix text being cleared/unset if only set via the IB editor.
• Fixes text alignments other than left set on the label rather than via NSParagraphStyle.
• Fixes VoiceOver element positions when placed inside a UIScrollView.