placeholders: add {currentUser}

[cottongin]

Placeholders

thelounge-plugin-shortcuts supports multiple placeholders:

• {currentChannel} will be replaced with the name of the channel where the shortcut was executed in
• {0}, {1}, {n} will be replaced with the first, second or nth argument that was entered after the shortcut
• {args} will be replaced with all arguments which were entered after the shortcut

It'd be great to also include a {currentUser} (or maybe {currentNickname} or similar) so we can make aliases for commands that target yourself, without hardcoding a nickname or using an {arg}.