Show a warning if the new crate would fail to be pushed to crates.io

[adelarsq]

When trying to use cargo new show a warning if the crate would fail to be pushed to crates.io.

This issue was based on this reddit topic.

[mattscamp]

Skimmed through the comments and the code associated. It seems like the only difference between them is crates.io required ASCII characters and some reserved names we should be checking for. Is that accurate?

[alexcrichton]

Thanks for the report! I think that this may actually be mostly a subset of #883, so I'm gonna close this in favor of that.

Yeah I think it'd be fine to change cargo new though to verify that the project name doesn't have ASCII by default, as that's mostly what crates.io does. I originally wanted to leave most of this validation to crates.io to avoid duplicating, but it should be small enough that it's harmless to duplicate.