add config-file preprocessor (%include)

docs/copyparty.d/foo/another.conf

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+# this file gets included twice from ../some.conf,
+# setting user permissions for a volume
+rw usr1
+r usr2
+% sibling.conf

docs/copyparty.d/foo/sibling.conf

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+# and this config file gets included from ./another.conf,
+# adding a final permission for each of the two volumes in ../some.conf
+m usr1 usr2

docs/copyparty.d/some.conf

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+# lets make two volumes with the same accounts/permissions for both;
+# first declare the accounts just once:
+u usr1:passw0rd
+u usr2:letmein
+
+# and listen on 127.0.0.1 only, port 2434
+-i 127.0.0.1
+-p 2434
+
+# share /usr/share/games from the server filesystem
+/usr/share/games
+/vidya
+# include config file with volume permissions
+% foo/another.conf
+
+# and share your ~/Music folder too
+~/Music
+/bangers
+% foo/another.conf
+
+# which should result in each of the volumes getting the following permissions:
+# usr1 read/write/move
+# usr2 read/move
+#
+# because another.conf sets the read/write permissions before it
+# includes sibling.conf which adds the move permission

docs/example2.conf

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+# 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