And implement the option init functions for this and the format options.

When we describe the workdir, we perform a describe on HEAD and then
check to see if the worktree is dirty. If it is and we have a suffix
string, we append that to the buffer.

Instead of printing out to the buffer inside the information-gathering
phase, write the data to a intermediate result structure.

This allows us to split the options into gathering options and
formatting options, simplifying the gathering code.

We don't describe arbitrary object, so let's give it the name of the one
object type we accept.

Provide a callback for certificate validation

The user may have the data hashed as MD5 or SHA-1, so we should provide
both types for consumption.

Instead of using the libssh2 defines, provide our own, which eases usage
as we do not need to check whether libgit2 was built with libssh2 or not.

Instead of spreading the data in function arguments, some of which
aren't used for ssh and having a struct only for ssh, use a struct for
both, using a common parent to pass to the callback.