Add example of linking against a C++ shared library

[domfarolino]

fixes #294 and contributes to #328.

Please take a look @budziq, I'd love for the cookbook to have a simple example of Rust dynamically linking against a C/C++ shared library through a C ABI.

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Things to check before submitting a PR

• the tests are passing locally with cargo xtask test all
  • Note: almost every test fails for me locally. I'm not sure why, but I'm pretty sure it is not related to my commits.
• commits are squashed into one and rebased to latest master
• PR contains correct "fixes #ISSUE_ID" clause to autoclose the issue on PR merge
• non rendered items are in sorted order (links, reference, identifiers, Cargo.toml)
  • Question: Sorted how? Lexicographically, or by use (i.e., link reference/use)
• links to docs.rs have wildcard version https://docs.rs/tar/*/tar/struct.Entry.html
• example has standard error handling
  • My liberal use of .unwrap() in basic helper code I think is OK here? But I'd love for someone to confirm!
• code identifiers in description are in hyperlinked backticks

[`Entry::unpack`]: https://docs.rs/tar/*/tar/struct.Entry.html#method.unpack