feat: support sharable configuration

Adds the options `extends`, which can be defined via configuration file or CLI arguments to a single path or an array of paths of shareable configuration.
A shareable configuration is a file or a module that can be loaded with `require`.
Options is defined by merging in the following order of priority:
- CLI/API
- Configuration file
- Shareable configuration (from right to left)

Options set in a shareable configuration can be unset by setting it to `null` or `undefined` in the main configuration file. If a default value applies to this property it will be used.

package.json

@@ -28,12 +28,12 @@
     "execa": "^0.8.0",
     "get-stream": "^3.0.0",
     "git-log-parser": "^1.2.0",
-    "import-from": "^2.1.0",
     "lodash": "^4.0.0",
     "marked": "^0.3.6",
     "marked-terminal": "^2.0.0",
     "p-reduce": "^1.0.0",
     "read-pkg-up": "^3.0.0",
+    "resolve-from": "^4.0.0",
     "semver": "^5.4.1"
   },
   "devDependencies": {