8.0.0

• [!BC] Minimum PHP Version 7.2.0
• [!BC] \FluentDOM\Loadable::load() now always returns
  \FluentDOM\Loader\Result instance (or NULL)
• [!BC] Define Return Type For \FluentDOM\Loadable::load()
• [!BC] Define Return Type For \FluentDOM\Loadable::loadFragment()
• [!BC] Define Appendable::appendTo() Return Value
  as "void"
• [!BC] Change Method Signatures Of DOM L3 Methods To Match PHP
• [!BC] Declare FluentDOM::setLoader() Argument As A Nullable
  Of Loadable
• [FEATURE] #90 Throw Exception If File Can Not Be Loaded
• [REFACTOR] Define Argument And Return Types
• [REFACTOR] Reference Loaders By Class Constants
• [REFACTOR] Cleanup Unit Tests