Component text can be truncated

- BaseComponentContext has new field terminalWidth.
- StringToStyleExpressionRenderer contains new format
  for "truncate-" prefixes and this is something
  what template can use to instruct max length based
  on terminal width.
- Change single/multi selectors to use this feature.
- Fixes #543

spring-shell-docs/src/main/asciidoc/using-shell-components-ui-render.adoc

@@ -89,3 +89,14 @@ from a parent component types. The following tables show those context variables
 |The current cursor row in a selector.
 
 |===
+
+
+[[componentcontext-template-variables]]
+.ComponentContext Template Variables
+|===
+|Key |Description
+
+|`terminalWidth`
+|The width of terminal, type is _Integer_ and defaults to _NULL_ if not set.
+
+|===