Rework command subsystem

- Focus of these changes are to introduce a new command system based on
  real registrations (new way) instead of continuously (old way) resolve
  methods and its parameters via reflection.
- There's a lot of changes as this resolution via reflection had its
  hooks almost everywhere and thus most changes are just refactorings.
- Order to understand real changes I'd start to look classes under
  `org.springframework.shell.command` package as it defines new registration,
  catalog and parser classes. Also samples contain new classes to demonstrate
  new functionality.
- Fixes #380

.gitignore

@@ -10,6 +10,8 @@ target/
 *.iws
 .DS_Store
 spring-shell.log
+shell.log
+shell.log.*.gz
 
 # Visual Studio Code
 .vscode/*