Expose new_sub_parser_from_file
Expose new_sub_parser_from_file

This function is useful when external tools like rustfmt want to parse
internal files without parsing a whole crate.

aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust