isolate repo fork command and tweak usage

This commit is another isolation refactor, this time for repo fork.
However, I got fed up with the --remote="true|false|prompt" style of
flags and took this opportunity to switch to a set of bool flags:

--remote and --clone
--no-remote and --no-clone

the string args were really non standard and confusing; with only two
bools it was impossible to tell when to prompt.

test/fixtures/forkResult.json

@@ -1,9 +0,0 @@
-{
-  "node_id": "123",
-  "name": "REPO",
-  "clone_url": "https://github.com/someone/repo.git",
-  "created_at": "2011-01-26T19:01:12Z",
-  "owner": {
-    "login": "someone"
-  }
-}