Add support for whitelisting environment variables to pass through (closes #76)

[dflemstr]

This adds a fairly straight-forward section to Cross.toml like so:

[build.env]
whitelist_all = true
# ...or...
whitelist = ["PATH", "RVM_VERSION", "LD_LIBRARY_PATH"]

[japaric]

Thank you for the PR, @dflemstr, and sorry for taking so long to review.

The implementation looks good to me but I'm a bit concerned about whitelist_all as that makes more likely to break the operation of Cross by overriding one of the env variables its docker container internally uses. Ideally, I'd like Cross to warn you if you were to override one of such internal variables but that would make the implementation of this feature way more complicated.

So, how about this: let's implement just whitelist and document this feature on the README noting that Cross requires some env variables to be set to certain values, perhaps also list the common ones in the README, to work correctly and that you should think twice about overriding those.

What do you think?

[kamalmarhubi]

I would like this to enable passing TravisCI env vars into the build. Let me know if I can help nudge this along!