String line to expand IRC tags of.
Whether or not to use extended colours (16-99).
Whether to expand tags or strip them from the input line.
The passed line but with tags expanded to formatting and colouring.
std.string.indexOf (used internally) throws UTFException if the starting index of a lookup doesn't represent a well-formed codepoint.
Implementation function for expandIRCTags. Kept separate so that UTFException can be neatly caught.