add config-file preprocessor (%include)

docs/example2.conf

@@ -0,0 +1,13 @@
+# you can include additional config like this
+# (the space after the % is important)
+#
+# since copyparty.d is a folder, it'll include all *.conf
+# files inside (not recursively) in alphabetical order
+# (not necessarily same as numerical/natural order)
+#
+# paths are relative from the location of each included file
+# unless the path is absolute, for example % /etc/copyparty.d
+#
+# max include depth is 64
+
+% copyparty.d