Add --no-comment-extraction option

[Evian-Zhang]

Nowadays, Rustdoc supports to add comment from separate markdown files by

#![doc = include_str!("README.md")]

As a result, it is great to combine this feature with cargo readme to eliminate the annoything # lines for GitHub README readers (motivation can be seen here).

The ideal workflow is:

1. User write a readme file README.rustdoc.md which is specific for rustdoc, and the lib.rs would include such readme file as documentation.

   In this file, code snippets are meant to be doc tested, and several # are added to hide lines.

2. Then, user would use cargo readme to process such file, and generate a README.md for the repository.

   In this README, lines hidden by # are removed, and badges, licenses, etc. are added as template do.

However, current cargo readme cannot do this, since the file specified in --input option would still be treated as a Rust file, and the documentation would be extracted by matching the comment patterns. As a result, I add the --no-comment-extraction option, which would treat input file as a plain markdown file, and no more extraction is needed.