add hook side-effects; closes #86

hooks can now interrupt or redirect actions, and initiate
related actions, by printing json on stdout with commands

mainly to mitigate limitations such as sharex/sharex#3992

xbr/xau can redirect uploads to other destinations with `reloc`
and most hooks can initiate indexing or deletion of additional
files by giving a list of vpaths in json-keys `idx` or `del`

there are limitations;
* xbu/xau effects don't apply to ftp, tftp, smb
* xau will intentionally fail if a reloc destination exists
* xau effects do not apply to up2k

also provides more details for hooks:
* xbu/xau: basic-uploader vpath with filename
* xbr/xar: add client ip

bin/hooks/reloc-by-ext.py

@@ -0,0 +1,94 @@
+#!/usr/bin/env python3
+
+import json
+import os
+import sys
+
+
+_ = r"""
+relocate/redirect incoming uploads according to file extension
+
+example usage as global config:
+    --xbu j,c1,bin/hooks/reloc-by-ext.py
+
+parameters explained,
+    xbu = execute before upload
+    j   = this hook needs upload information as json (not just the filename)
+    c1  = this hook returns json on stdout, so tell copyparty to read that
+
+example usage as a volflag (per-volume config):
+    -v srv/inc:inc:r:rw,ed:c,xbu=j,c1,bin/hooks/reloc-by-ext.py
+                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+    (share filesystem-path srv/inc as volume /inc,
+     readable by everyone, read-write for user 'ed',
+     running this plugin on all uploads with the params explained above)
+
+example usage as a volflag in a copyparty config file:
+    [/inc]
+      srv/inc
+      accs:
+        r: *
+        rw: ed
+      flags:
+        xbu: j,c1,bin/hooks/reloc-by-ext.py
+
+note: this only works with the basic uploader (sharex and such),
+  does not work with up2k / dragdrop into browser
+
+note: this could also work as an xau hook (after-upload), but
+  because it doesn't need to read the file contents its better
+  as xbu (before-upload) since that's safer / less buggy
+"""
+
+
+PICS = "avif bmp gif heic heif jpeg jpg jxl png psd qoi tga tif tiff webp"
+VIDS = "3gp asf avi flv mkv mov mp4 mpeg mpeg2 mpegts mpg mpg2 nut ogm ogv rm ts vob webm wmv"
+MUSIC = "aac aif aiff alac amr ape dfpwm flac m4a mp3 ogg opus ra tak tta wav wma wv"
+
+
+def main():
+    inf = json.loads(sys.argv[1])
+    vdir, fn = os.path.split(inf["vp"])
+
+    try:
+        fn, ext = fn.rsplit(".", 1)
+    except:
+        # no file extension; abort
+        return
+
+    ext = ext.lower()
+
+    ##
+    ## some example actions to take; pick one by
+    ## selecting it inside the print at the end:
+    ##
+
+    # create a subfolder named after the filetype and move it into there
+    into_subfolder = {"vp": ext}
+
+    # move it into a toplevel folder named after the filetype
+    into_toplevel = {"vp": "/" + ext}
+
+    # move it into a filetype-named folder next to the target folder
+    into_sibling = {"vp": "../" + ext}
+
+    # move images into "/just/pics", vids into "/just/vids",
+    # music into "/just/tunes", and anything else as-is
+    if ext in PICS.split():
+        by_category = {"vp": "/just/pics"}
+    elif ext in VIDS.split():
+        by_category = {"vp": "/just/vids"}
+    elif ext in MUSIC.split():
+        by_category = {"vp": "/just/tunes"}
+    else:
+        by_category = {}
+
+    # now choose the effect to apply; can be any of these:
+    # into_subfolder  into_toplevel  into_sibling  by_category
+    effect = into_subfolder
+    print(json.dumps({"reloc": effect}))
+
+
+if __name__ == "__main__":
+    main()