feat create namespace

[ashleymichal]

Closes #340 . Includes commit from #391 (merge first).

This is dependent on an update to the cloudflare-rs library per this issue: cloudflare/cloudflare-rs#10.

Interacting with this code looks a little something like this:
On successful creation:

$ wrangler kv create "my namespace"
🌀  Creating namespace with title "my namespace"
✨  Success: WorkersKVNamespace {
    id: "f7b02e7fc70443149ac906dd81ec1791",
    title: "my namespace",
}

On API error (in this case duplicate kv namespace):

$ wrangler kv create "my namespace"
🌀  Creating namespace with title "my namespace"
⚠️  Error 10014: a namespace with this account ID and title already exists
🕵️‍♂️  Run `wrangler kv list` to see your existing namespaces with IDs