Provides a way of passing a custom ssl context to the connection.

[mzimbres]

@anarthal It would be great if you had a look at this PR. Thanks.

[anarthal]

It looks like moving a connection constructed from a null context leaves a dangling pointer to the fallback context. Plus you're creating a context per object even if you don't need one. If you're gonna do like that, better pass the context by value.

I solved both of these issues by having the fallback context as a singleton. Klemens solved it using an Asio service.

[mzimbres]

@mzimbres I am glad I pulled you in the PR. I have been hurrying with some implementations as my free time has dropped.

[anarthal]

I also recommend using ssl::context::tlsv12_client as the default method. This disables support for TLS 1.0 and 1.1, which are considered vulnerable.