Support include_str! in crate-level doc attributes

[schneems]

Context

Using include_str! is a great way to keep docs DRY (single point of truth) and up-to-date. The same contents can be reused in multiple places, and the contents of the file can be used in assertions in unit tests as well.

One use case is my crate https://crates.io/crates/path_facts. It is used to display facts about a path on disk. To make a consistent output, in tests, I need to create a temp dir, and manipulate it. But this produces unpredictable output as the tempdir changes every time. In unit tests, I have helpers that normalize these values so the output renders consistently:

Path does not exist `/path/to/directory/a.txt/b/c/does_not_exist.txt`
                                        ^^^^^
                                        ↳ File, not a dir [✅ read, ✅ write, ❌ execute]

I want to show this in my docs as well, but the sanitization infrastructure isn't part of the public interface (and doc tests require you to import only public interfaces). So one solution is to use those contents via an include_str! in my unit tests, and then to render the contents in my docs.

I can use this approach everywhere EXCEPT for my lib.rs module docs, since I'm using cargo-rdme to keep my README DRY.

This PR

Adds support for #![doc = include_str!("<path>")] (only, no other macros) to cargo-rdme. It does this by parsing the directive and special casing doc = include_str and then re-implementing the (relatively simple) macro logic to pull in the contents of that file.

Closes #178

[schneems]

Closing in favor of #297 which supports ALL macros and is (therefore) a little less hacky.