Implement possibility to check for existing config properties

[codablock]

Right now, it's only possible to get a config value, which will always return the default value for that property in case it was not set in the config. This makes it impossible to act differently on non-existing entries. As an example, the port should not default to 22 when it was passed as part of the ssh invocation.

An incarnation of this issue can be found in go-git.

[kke]

I would like to know if the IdentityFiles returned are all system defaults or explicitly set for the host / match.