Add support for exit codes

- New configurations to CommandRegistration
- Re-using exit code concepts from boot
- Handling exit codes only in non-interactive mode
- Adding e2e commands and tests for better coverage
- Fixes #431

spring-shell-docs/src/main/asciidoc/using-shell-commands-exitcode.adoc

@@ -0,0 +1,39 @@
+[[dynamic-command-exitcode]]
+==== Exit Code
+ifndef::snippets[:snippets: ../../test/java/org/springframework/shell/docs]
+
+Many command line applications when applicable return an _exit code_ which running environment
+can use to differentiate if command has been executed successfully or not. In a `spring-shell`
+this mostly relates when a command is run on a non-interactive mode meaning one command
+is always executed once with an instance of a `spring-shell`.
+
+Default behaviour of an exit codes is as:
+
+- Errors from a command option parsing will result code of `2`
+- Any generic error will result result code of `1`
+- Obviously in any other case result code is `0`
+
+Every `CommandRegistration` can define its own mappings between _Exception_ and _exit code_.
+Essentially we're bound to functionality in `Spring Boot` regarding _exit code_ and simply
+integrate into that.
+
+Assuming there is an exception show below which would be thrown from a command:
+
+====
+[source, java, indent=0]
+----
+include::{snippets}/ExitCodeSnippets.java[tag=my-exception-class]
+----
+====
+
+It is possible to define a mapping function between `Throwable` and exit code. You can also
+just configure a _class_ to _exit code_ which is just a syntactic sugar within configurations.
+
+====
+[source, java, indent=0]
+----
+include::{snippets}/ExitCodeSnippets.java[tag=example1]
+----
+====
+
+NOTE: Exit codes cannot be customized with annotation based configuration

spring-shell-docs/src/main/asciidoc/using-shell-commands.adoc

@@ -18,3 +18,5 @@ include::using-shell-commands-programmaticmodel.adoc[]
 include::using-shell-commands-organize.adoc[]
 
 include::using-shell-commands-availability.adoc[]
+
+include::using-shell-commands-exitcode.adoc[]