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

docs/versus.md

@@ -175,6 +175,7 @@ symbol legend,
 | ┗ randomize filename    | █ |   |   |   |   |   |   | █ | █ |   |   |   |   |
 | ┗ mimetype reject-list  | ╱ |   |   |   |   |   |   |   | • | ╱ |   | ╱ | • |
 | ┗ extension reject-list | ╱ |   |   |   |   |   |   | █ | • | ╱ |   | ╱ | • |
+| ┗ upload routing        | █ |   |   |   |   |   |   |   |   |   |   |   |   |
 | checksums provided      |   |   |   | █ | █ |   |   |   | █ | ╱ |   |   |   |
 | cloud storage backend   | ╱ | ╱ | ╱ | █ | █ | █ | ╱ |   |   | ╱ | █ | █ | ╱ |
 
@@ -188,6 +189,9 @@ symbol legend,
 
 * `race the beam` = files can be downloaded while they're still uploading; downloaders are slowed down such that the uploader is always ahead
 
+* `upload routing` = depending on filetype / contents / uploader etc., the file can be redirected to another location or otherwise transformed; mitigates limitations such as [sharex#3992](https://github.com/ShareX/ShareX/issues/3992)
+  * copyparty example: [reloc-by-ext](https://github.com/9001/copyparty/tree/hovudstraum/bin/hooks#before-upload)
+
 * `checksums provided` = when downloading a file from the server, the file's checksum is provided for verification client-side
 
 * `cloud storage backend` = able to serve files from (and write to) s3 or similar cloud services; `╱` means the software can do this with some help from `rclone mount` as a bridge