API for connecting a client to an existing server by name

It's not currently possible to create a `Vimrunner::Server` instance (and therefore `Vimrunner::Client` connected to it) that represents an existing, running Vim server.

This means that it is not possible to start a Vim server independently of Vimrunner and then control it remotely.

We could consider this out of scope but perhaps Vimrunner's remit is to provide an abstraction on top of Vim's remote functionality which should include such a method of interaction.

Perhaps something like:

```console
$ vim --servername FOO
```

```ruby
vim = Vimrunner.connect("FOO")
vim.insert("Hello world!")
```