irc: add tag with host ("host_xxx") in IRC messages displayed
trigger: add variable "tg_tag_host" (from tag "host_xxx" in message)
core: add tag "host_xxx" in /help filter
core: add note about API mismatch (rmodifier.so) in release notes
charset: fix /help charset.default.decode

The charset is used only when the message is not UTF-8 valid.
core: use lower case for CMake keywords
core: fix crash on "weechat --upgrade" if no .upgrade files are found
api: allow wildcard "*" inside the mask in function string_match