Handle alt+enter as newline char

Let user print newline character using alt+enter keybind,
it allows great flexibility in communication,
now user will be able to write much better longer messages.

docs/profanity.1

@@ -80,6 +80,9 @@ Mark current window for later reading with an attention flag.
 .BI ALT+m
 Switch between windows marked with the attention flag.
 .TP
+.BI ALT+ENTER
+Add newline character without sending a message/command.
+.TP
 .BI ALT+c
 Run external editor (see
 .BR profanity-editor (1))