Add initial support for XEP-0363. HTTP uploads can be used when colon
separated values for stdin is enabled:
~~~
u::::<filename>|<size (bytes)>[|<content-type>]
u::::example.png%7C16698
u::::tr.png%7C16698%7Cimage%2Fpng
~~~
The reponse:
~~~
U:upload.example.com:user@example.com/477937350262208314215778:https%3A%2F%2Fexample.com%2Fupload%2F1234%2Fabc%2Fexample.png%7Chttps%3A%2F%2Fexample.com%2Fupload%2F1234%2Fabc%2Fexample.png
~~~
TODO:
* support PUT header elements
* support/test error conditions
Questions:
* save the maximum file size returned by the server and disallow uploads
larger than the value?
* xmppipe is "pinned" to the upload server returned in the IQ reply (the
"to" field is ignored)
* allow other upload servers?
* error if different upload server is specified in "u:<from>:<to>"?
Michael Santos