Update code

[theblixguy]

This PR updates the code with the following changes:

1. Swift 4.2 support.
2. Fixed indentation -> Width: tab = 2 & indent = 2.
3. Make classes final.
4. Add guards to check if a URL is valid and if a value can successfully be casted to a Bool.
5. Make enum keys lowercase.
6. Turn on Whole Module Optimisation and other recommended warnings.
7. Remove unnecessary parentheses from switch and switch statements.
8. Use [] instead of Dictionary<>.