Add individual write subcommand for Workers KV

[gabbifish]

This PR closes #343.

The SET behavior is used the following way:

Success case (set string value):

$ wrangler kv set-key 06779da6940b431db6e566b4846d64db taylor swift --ttl 60
 Success

Success case (set value from file contents)

$ wrangler kv write-key 06779da6940b431db6e566b4846d64db my_file abc/a.txt --ttl 60 -f
 Success

Failure cases

$ wrangler kv write-key 06779da6940b431db6e566b4846d63db --ttl 60 -f my_file abc/a.txt
 Error 10013: namespace not found
 Run `wrangler kv list` to see your existing namespaces with IDs

$ wrangler kv write-key 06779da6940b431db6e566b4846d64db --ttl 60 -f my_file abc/ab.txt
Error: No such file or directory (os error 2)

^ There is the standard emoji logic here, it just doesn't show up in Ubuntu for now...

In a future PR, we'll use cloudflare-rs for this; its usage is unfortunately blocked due to cloudflare/cloudflare-rs#27 (cloudflare-rs doesn't support non-json API responses, and the GET operation for Workers KV keys returns raw text on success).

[ashleymichal]

this command should be "write"