Feature request: Option to add crates.io badge

[thanadolps]

The automatic badge from cargo.toml has been very helpful to me so far, not having to do tedious search, copy, and paste is amazing.
However there are one badge that many project use but aren't included in Cargo.toml's [badges], a crates.io badge.

Option to add crates.io badge would be very helpful for many project and doesn't seem difficult to add since project's name are in Cargo.toml.

[livioribeiro]

That's interesting, I will look at it as soon as I can

[fosskers]

Adding this would be similar to this commit, yeah?

[BartMassey]

I started to throw together a PR. One issue is that this badge should probably only be used if published is not false in the Cargo manifest. Will ignore for now, but feedback welcome.