FR: allow substitution templates

Instead of replacing with the text to the right of the `=` sign, it could generate the replacement text from a configurable template which can optionally also use the original nickname. For example, the template (which would be an additional setting) could have values like `%name (%nick)` so that if the mapping contained a line:

jbloggs=Joe Bloggs

then the replacement text would be `Joe Bloggs (jbloggs)`. It could even get fancy and have conditional formatting based on the lengths of `%name` and `%nick`.

Start by locating the userscript's nickname mapping, replacement logic, and settings handling. Define how a configurable template expands %name and %nick, including the requested conditional formatting, then verify that mapped nicknames produce the specified output and that existing replacements still work.

Written by the indexing model from the issue text.