Ciphersuite selection philosophy

In #56 we're working towards exporting ciphersuites so that servers can select the ones they want to support. However, we should consider if this is a feature of the rustls API that we want to implement. The rustls default is to offer all of the ciphersuites it supports, which is a good, modern selection without known issues. So the purpose of configuring custom ciphersuites would presumably be to disable a ciphersuite that is found to be broken at some point in the future. However, if that turns out to be the case, rustls would (presumably) ship a new release removing that ciphersuite, and crustls would do that same; so the fix, like fixes for other CVEs, would be to upgrade your libraries.

One argument in favor of supporting configuration of ciphersuites is that rustls supports it, and since we are a translation layer for rustls, we should support it to. But crustls is (for now) a subset of rustls and we focus on implementing the features that are most needed.

Adam Langley has a post about cryptographic agility and its cost here: https://www.imperialviolet.org/2016/05/16/agility.html.

/cc @icing

Start by reading issue #56, the rustls API discussion, and the linked post on cryptographic agility. Determine whether crustls should expose ciphersuite selection or retain its current subset, then record a clear decision and rationale in this issue; no implementation scope is defined yet.