ENH: support default values in string expansion

- syntax as per Bourne/Korn shell
      ${parameter:-defValue}

  If parameter is unset or null, the \c defValue is substituted.
  Otherwise, the value of parameter is substituted.

applications/test/string/Test-string.C

@@ -40,8 +40,9 @@ int main(int argc, char *argv[])
 {
     string test
     (
-        "  $HOME kjhkjhkjh \" \\$HOME/tyetyery ${FOAM_RUN} \n ; hkjh ;$ with "
-        " $(DONOTSUBST) some other ${USER} entries   "
+        "  $HOME kjhkjhkjh \" \\$HOME/tyetyery $; ${FOAM_RUN} \n $; hkjh;"
+        " $(DONOTSUBST) some other <${USER}> with '${__UNKNOWN:-some default}'"
+        " value "
     );
 
     dictionary dict;