Add support for per-project custom DSL sloc

[evan0greenup]

Just like languages.json, It define the language tokei support.

It would be nice to add .tokei.yaml or .sloc.yaml for in the top directory of repository. It following the same structure as languages.json but in YAML for defining user custom language.

This feature is useful especially in Rust ecosystem.

Because sometimes user create procedural macro and use custom DSL in separated files to generate Rust code.

[evan0greenup]

@XAMPPRocky , what is your opinion?