Resolves#1216.
* Add support for spoiler formatting
* Implement support for sending message attachments that are spoilers
* use consistent naming
* update docstring for new isSpoiler param
* move extension method to be under core project, make spoiler prefix a const
* typo fix
* update missing xmldocs
* move SpoilerPrefix const outside of interface
* Add isSpoiler support to webhook client
adds the isSpoiler field to uploading files with a webhook, which will only
insert "SPOILER_" to the start of the filename. This does not include other
fields in the payload, as this is not in the documentation, and was not observed
like in the regular client
f3b20b2b6d