Bug fixes, improved link handling and Swift 5 support

[levi]

• Added support for both URL and string links, as the mutable string API supports both and it's handy to have links that point to an identifier, rather than a valid URL.
• Fixed a bug where using new line breaks would sometimes not allow a link to be tappable (text container was not being sized correctly, which invisibly cut off the text from the character at point query).
• Expanded the default initializer to allow for custom default attributes. Allows for making subclasses that provided default values for larger projects.

Includes Swift 4 and 5 support added by @numen31337 and @fespinoza.