Add pre-build hook

[andrewbanchich]

I'd like to use an official image like centos/devtoolset-7-toolchain-centos7, which I can do by adding to my Cross.toml. However, the image doesn't come with openssl installed. It seems like overkill to have to create and maintain a Dockerfile, then add an extra build script to my CI just to get openssl.

Instead, would it be possible to do something like this?

[target.x86_64-unknown-linux-gnu]
image = "centos/devtoolset-7-toolchain-centos7"
pre-build = "yum install openssl-devel"

[Emilgardis]

This has been implemented!

[target.x86_64-unknown-linux-gnu]
image = "centos/devtoolset-7-toolchain-centos7"
pre-build = ["yum install openssl-devel"]

should just work now with latest cross :)