feat(medusa-dev-cli): adds helpers to manage feature flags (#1770)

**Usage**

**Create a new feature flag**
```
$ medusa-dev ff create [name of flag] -d [description of what flag is for]
```
Will put a new file in `packages/medusa/src/loaders/feature-flags/[kebab-cased-flag-name].ts` and fill out the details.

**List feature flags**
```
$ medusa-dev ff list
```
Note: your Medusa repo must be built for the flags to show up

**Delete a feature flag**
```
$ medusa-dev ff delete [name of flag]
```
Will delete a file at `packages/medusa/src/loaders/feature-flags/[kebab-cased-flag-name].ts` if it exists.

packages/medusa-dev-cli/package.json

@@ -29,9 +29,7 @@
     "cross-env": "^7.0.3"
   },
   "homepage": "https://github.com/medusajs/medusa/tree/master/packages/medusa-dev-cli#readme",
-  "keywords": [
-    "medusa"
-  ],
+  "keywords": ["medusa"],
   "license": "MIT",
   "main": "index.js",
   "repository": {