Enable multiline input

Adds a nonl() call to avoid translation from \r to \n when pressing
enter.
C-J -> \n
Enter -> \r
To allow multiline input, \n chars (e.g. when pressing C-J) are replaced
with a pilcrow. After hitting enter, every pilcrow is substituted with
\n again.

src/misc_tools.c

@@ -176,6 +176,15 @@ int string_is_empty(const char *string)
     return string[0] == '\0';
 }
 
+/* Returns 1 if the string is empty, 0 otherwise */
+int wstring_is_empty(const wchar_t *string)
+{
+    if (!string)
+        return true;
+
+    return string[0] == L'\0';
+}
+
 /* convert a multibyte string to a wide character string and puts in buf. */
 int mbs_to_wcs_buf(wchar_t *buf, const char *string, size_t n)
 {