[Feature Request] Allow `line_length_guide` on a per-extension basis

[lberrymage]

In config.yml, only tab_width and soft_tabs can be configured per file extension. I would personally like to see line_length_guide per-extension configurable as well, considering different languages' conventions, e.g. rustfmt's default line length is 100, whereas most others have a conventional max length of 80. This is somewhat related to the project card referencing formatter integration, as deducing the line_length_guide from formatter configs would be a convenient addition. However, I suggest that deducing the value be addressed in a different issue.