Lexer value - lexer-language

[prettydiff]

This is not a big change to the project, but would change the output enough to warrant a new major version number.

Currently the lexer value is the lexer name. This issue will change it to be lexer-language, such as markup-html or script-typescript.

The rationale for this issue is flexibility.

1. Provide the ability to switch between languages using the same lexer file.
2. Provide contributors the ability to create alternate lexer files for existing supported languages