Auto merge of #2917 - alexcrichton:cainfo, r=brson

Add `http.cainfo` config for custom certs

This adds a `http.cainfo` option to Cargo which reads CA information from a
bundle to pass through to the underlying SSL implementation. This should allow
configuration of Cargo in situations where the default certificate store doesn't
contain the relevant certificates, such as behind corporate proxies.

cc #1180