Add base shell test system

- NOTE: very much wip and unstable
- This commit is a first step to provide boot style
  @ShellTest annotation
- New modules spring-shell-test and spring-shell-test-autoconfigure
- Focus is to autoconfigure context without shell runners so that
  we can create "sessions" and hook to configures jline terminal
  with custom in/out streams.
- Skeleton fork from jediterm to provide basic terminal emulation
  to part of a control amd escape characters working.
- ShellTestClient is a concept user can use to interact with a shell
  in a same way user would use a "real" shell.
- Fixes #489

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

@@ -0,0 +1,18 @@
+[[using-shell-testing]]
+== Testing
+ifndef::snippets[:snippets: ../../test/java/org/springframework/shell/docs]
+
+Testing cli application is difficult due to various reasons:
+
+- There are differences between OS's.
+- Within OS there may be different shell implementations in use.
+- What goes into a shell and comes out from a shell my be totally
+  different what you see in shell itself due to control characters.
+- Shell may feel syncronous but most likely it is not meaning when
+  someting is written into it, you can't assume next update in
+  in it is not final.
+
+NOTE: Testing support is currently under development and will be
+      unstable for various parts.
+
+include::using-shell-testing-basics.adoc[]