pr command scriptability improvements

This commit improves behavior and error handling for pr commands when
run unattached to a tty.

- error if pr create and no -t/-f
- error if pr create and -w
- machine readable pr list
- machine readable pr view
- various cleanup of informational messages

test/fixtures/prList.json

@@ -9,7 +9,9 @@
               "number": 32,
               "title": "New feature",
               "url": "https://github.com/monalisa/hello/pull/32",
-              "headRefName": "feature"
+              "headRefName": "feature",
+              "state": "OPEN",
+              "isDraft": true
             }
           },
           {
@@ -18,6 +20,8 @@
               "title": "Fixed bad bug",
               "url": "https://github.com/monalisa/hello/pull/29",
               "headRefName": "bug-fix",
+              "state": "OPEN",
+              "isDraft": false,
               "isCrossRepository": true,
               "headRepositoryOwner": {
                 "login": "hubot"
@@ -27,6 +31,8 @@
           {
             "node": {
               "number": 28,
+              "state": "MERGED",
+              "isDraft": false,
               "title": "Improve documentation",
               "url": "https://github.com/monalisa/hello/pull/28",
               "headRefName": "docs"
@@ -40,4 +46,4 @@
       }
     }
   }
-}
+}