Add err parameter to echo

clikt/src/main/kotlin/com/github/ajalt/clikt/output/TermUi.kt

@@ -13,11 +13,13 @@ object TermUi {
      * This is similar to [print] or [println], but converts newlines to the system line separator.
      *
      * @param message The message to print.
-     * @param trailingNewline if true, behave like [println], otherwise behave like [print]
+     * @param trailingNewline If true, behave like [println], otherwise behave like [print]
+     * @param err If true, print to stderr instead of stdout
      */
-    fun echo(message: Any?, trailingNewline: Boolean = true) {
+    fun echo(message: Any?, trailingNewline: Boolean = true, err: Boolean = false) {
         val text = message?.toString()?.replace(Regex("\r?\n"), System.lineSeparator()) ?: "null"
-        if (trailingNewline) println(text) else print(text)
+        val ps = if (err) System.err else System.out
+        if (trailingNewline) ps.println(text) else ps.print(text)
     }
 
     /**

docs/_data/sidebars/clikt_sidebar.yml

@@ -9,7 +9,7 @@ entries:
 
     - title: Basic Concepts
       url: /quickstart.html#basic-concepts
-    - title: Echoing
+    - title: Printing to Stdout and Stderr
       url: /quickstart.html#echoing
     - title: Nesting Commands
       url: /quickstart.html#nesting-commands

docs/pages/quickstart.md

@@ -42,7 +42,7 @@ Options:
   -h, --help  Show this message and exit
 ```
 
-## Echoing
+## Printing to Stdout and Stderr {#echoing}
 
 Why does this example use {% include api.html pkg="output"
 class="term-ui" fun="echo" %} instead of
@@ -52,7 +52,8 @@ support. {% include api.html pkg="output" class="term-ui" fun="echo"
 text="echo"%} automatically translates line breaks into the line
 separator for the current platform. So you don't have to worry that some
 of your users will see mangled output because you didn't test on
-Windows.
+Windows. You can also pass `err=true` to `echo` to print to stderr
+instead of stdout.
 
 ## Nesting Commands