Add long press gesture support

[jamztang]

Particular useful for use case that needed to perform different actions based on user actions.

Say normal tap goes to in-app browser, long pressing shows a dialog for copy link and open in Safari.

pod ‘TTTAttributedLabel’, :git => ‘https://github.com/jamztang/TTTAttributedLabel.git’, :branch => ‘longpresssupport’

[getaaron]

@jamztang I just submitted a similar pull request here: #302 - I'm curious if you have any feedback on it. I think I like your performSelector: approach over my NSTimer approach. I took the approach of adding separate delegate methods instead of a state variable. I'm not sure which of those I like better yet.

Two differences I noticed:

• One thing I ran into in my implementation is that a user might press on one link, then slide onto a different link, but the first link would still be registered. (See my handling in touchesMoved:withEvent:.)
• I'm exposing a CGPoint when a user long-presses on a link. This is helpful for iPad apps, in which a UIActionSheet must be presented from a specific rect.

[jamztang]

I have looked into your implementation at #270 before. I wanted to do minimal changes to both the library (minimal additional lines) and developers (just one condition to adopt the long press, instead of needing to implement a new method), yet customisable to disable the long press gesture easily, and also extensible to add more info like the CGPoint info to just another property when needed.

I think the performSelector is a bit more easier to control and simpler to just cancel right away when touch ended or cancelled.

[mattt]

Thanks for your PR, @jamztang. Of the two current proposals for this feature, I prefer #302, though I'm not entirely sold on that particular implementation either. Defining an enum for tap interactions and exposing a long press interval are not patterns I'd be interested in pulling in.