feat: adhere to ssh_config more strictly

`ssh_config` allows multiple `Host` patterns to match a provided
hostname. When multiple blocks match, the first value of any `key` is
used.

Furthermore, some host patterns can be negated, in which case, when a
host matches that block is ignored entirely.

Also, `IdentityFile` is a bit different from other parameters: every
instance adds a value to the list of files.

This PR adds a more strict parsing to config files into an `SshConfig`
struct which can then be queried for a specific host. The configuration
is merged according to the rules above.

The new types remain private to the config module, but they could
eventually be made public so clients can implement more ssh2 features.

Author: plaflamme