StringInput masking

- Add a feature to define a mask character for StringInput
  which help when there's a need to as something sensitive.
- This masks both input and result value.

spring-shell-core/src/main/resources/org/springframework/shell/component/string-input-default.stg

@@ -1,9 +1,17 @@
 // info section after '? xxx'
 info(model) ::= <%
-<if(model.input)>
-<model.input>
+<if(model.hasMaskCharacter)>
+  <if(model.maskedInput)>
+    <model.maskedInput>
+  <else>
+    <("[Default "); format="value"><model.defaultValue; format="value"><("]"); format="value">
+  <endif>
 <else>
-<("[Default "); format="value"><model.defaultValue; format="value"><("]"); format="value">
+  <if(model.input)>
+    <model.input>
+  <else>
+    <("[Default "); format="value"><model.defaultValue; format="value"><("]"); format="value">
+  <endif>
 <endif>
 %>
 
@@ -14,7 +22,7 @@ question_name(model) ::= <<
 
 // component result
 result(model) ::= <<
-<question_name(model)> <model.resultValue; format="value">
+<question_name(model)> <model.maskedResultValue; format="value">
 >>
 
 // component is running