[FEATURE REQUEST] Support passing dictionary names instead of ids in dictionary-entry

[gsson]

We make extensive use of dictionaries in our Fastly services. We use different services for different environments, but we use the same consistent dictionary names across all environments.

One major pain point of using the CLI to manage entries is that the dictionary id varies across services, meaning we need to look up the dictionary id before being able to eg. list the items.

The workflow goes something like this:

$ FASTLY_API_TOKEN=...
$ fastly dictionary describe --version=active --service-name='our-service-name'  --name='our-dictionary-name'
...
ID: the-dictionary-id
...
$ fastly dictionary-entry list --service-name='out-service-name' --dictionary-id='the-dictionary-id'
...

It would simplify day-to-day usage to be able to just do:

$ FASTLY_API_TOKEN=...
$ fastly dictionary-entry list --version=active --service-name='out-service-name' --dictionary-name='our-dictionary-name'
...

Similar to #188