Generalized include_dir to accept multiple args

[darthdeus]

I'm not sure if this PR should even be merged as is, I'm partially opening it in case someone else runs into this as well.

The issue being when you want to call include_dir from another macro and compose the path, e.g. as

include_dir!("foo/" $bar "/baz");

[kolbma]

@darthdeus Maybe you should rebranch your PR without changing crate names!

But I'd love to get this into include_dir. Not sure why there is this limitation to a fixed single string?!

@Michael-F-Bryan Do you have any comments?

[darthdeus]

Oops sorry I had to rename the crate in order to publish comfy-core. I completely forgot I made this PR, and when I wanted to release comfy-core that has a dependency on this PR I ran into the issue that crates.io won't let you depend on github.

The only apparent solution being "republish as a new crate", which is what I did in order to be able to publish my crate.