Mapping file extensions to existing language via the command line

[rotty]

I'm working on a DSL that uses Scheme syntax, but I currently use a non-Scheme file extension to differentiate the files from normal Scheme code. It would be cool if I could tell tokei to treat those files as Scheme code, as the syntax is the same. As my DSL currently has approximatly one user (myself), it would be overly bold to add it as a Scheme extension inside languages.json. However, if I could extend a languages' extension list via the command line, I would be able to use tokei to count the amount of code in my DSL in my project without adding a niche language extension to the tokei codebase. I would suggest something like this (bikeshedding alert, the proposed option name is just a stand-in):

tokei --add-extension Scheme:bmx

When running tokei like this, it would classify .bmx files as Scheme code. What do you think about the general idea? If something like this is deemed a useful addition to the codebase, I might try my hand at a PR. This proposal is somewhat related to PR #1262; if the addition of the file extensions proposed in that PR would be deemed to be too niche to be added to language.json, the intent of the PR could still be fulfilled by using my proposed CLI extension.