feat: drop cross-fetch polyfill dep (#127)

Co-authored-by: Jason Kuhrt <jason.kuhrt@dialogue.co>

BREAKING CHANGE:

You are responsible for polyfilling your environment with `cross-fetch` if needed now.

package.json

@@ -36,9 +36,7 @@
     "test": "npm run lint && npm run build && ava --serial && npm run size",
     "size": "bundlesize"
   },
-  "dependencies": {
-    "cross-fetch": "2.2.2"
-  },
+  "dependencies": {},
   "devDependencies": {
     "@types/fetch-mock": "5.12.2",
     "@types/node": "8.5.5",

src/index.ts

@@ -1,7 +1,5 @@
 /// <reference lib="dom" />
 
-import 'cross-fetch/polyfill'
-
 import { ClientError, GraphQLError, Variables } from './types'
 
 export { ClientError } from './types'

yarn.lock

@@ -1013,13 +1013,6 @@ create-error-class@^3.0.0:
   dependencies:
     capture-stack-trace "^1.0.0"
 
-cross-fetch@2.2.2:
-  version "2.2.2"
-  resolved "https://registry.yarnpkg.com/cross-fetch/-/cross-fetch-2.2.2.tgz#a47ff4f7fc712daba8f6a695a11c948440d45723"
-  dependencies:
-    node-fetch "2.1.2"
-    whatwg-fetch "2.0.4"
-
 cross-spawn@^4.0.0:
   version "4.0.2"
   resolved "https://registry.yarnpkg.com/cross-spawn/-/cross-spawn-4.0.2.tgz#7b9247621c23adfdd3856004a823cbe397424d41"
@@ -2192,10 +2185,6 @@ node-abi@^2.2.0:
   dependencies:
     semver "^5.4.1"
 
-node-fetch@2.1.2:
-  version "2.1.2"
-  resolved "https://registry.yarnpkg.com/node-fetch/-/node-fetch-2.1.2.tgz#ab884e8e7e57e38a944753cec706f788d1768bb5"
-
 node-fetch@^1.3.3:
   version "1.7.0"
   resolved "https://registry.yarnpkg.com/node-fetch/-/node-fetch-1.7.0.tgz#3ff6c56544f9b7fb00682338bb55ee6f54a8a0ef"
@@ -3366,10 +3355,6 @@ well-known-symbols@^1.0.0:
   version "1.0.0"
   resolved "https://registry.yarnpkg.com/well-known-symbols/-/well-known-symbols-1.0.0.tgz#73c78ae81a7726a8fa598e2880801c8b16225518"
 
-whatwg-fetch@2.0.4:
-  version "2.0.4"
-  resolved "https://registry.yarnpkg.com/whatwg-fetch/-/whatwg-fetch-2.0.4.tgz#dde6a5df315f9d39991aa17621853d720b85566f"
-
 which-pm-runs@^1.0.0:
   version "1.0.0"
   resolved "https://registry.yarnpkg.com/which-pm-runs/-/which-pm-runs-1.0.0.tgz#670b3afbc552e0b55df6b7780ca74615f23ad1cb"