Write formatter spec

[zimbatm]

Describe what interface a formatter should have.

We make a few assumptions about formatters, which allows us to keep a simple design. So we should communicate to users and formatters out there what they are.

1. prjfmt is responsible for traversing the filesystem.
2. The formatter is a command-line tool that takes a list of files to format at the end of the argument list. Eg: rustfmt --edition 2018 src/main.rs src/lib.rs src/other.rs. Where --edition 2018 is an option, and src/main.rs src/lib.rs and src/other.rs are the files to format.
3. The formatter reads each of the files, formats them, and writes them back to the original location. The writing only happens if the file content has changed.

[zimbatm]

Eg: #30 doesn't respect that interface.