Some enhancements and filters

[vburlak]

• [x] [Fix] public_base_url according on class position
• [x] [Enhancement] Replace class init to after_setup_theme action. Now we can easily extend Envato_Theme_Setup_Wizard class in out theme without editing parent.
• [x] [Feature] Add filter $theme_name.'_theme_setup_wizard_username'.
• [x] [Feature] Add filter $theme_name.'_theme_setup_wizard_oauth_script'.
• [x] [Feature] Added page_slug. Now we can customize setup wizard url
• [x] Added version property.
• [ ] Add example.php with filters and class extending example.

[vburlak]

Here is some improvements, that should be helpful for everyone who let use this setup wizard. @dtbaker , I think we need create some roadmap for future enhancements and fixes (such as #5) Cheers!

[dtbaker]

Thanks! :+1: