refactor: define stylesheet paths in npm config

Define an npm config variable in package.json to specify where the
stylesheets are that we want to lint and update our package scripts to
reference this.

package.json

@@ -16,9 +16,9 @@
     "lint-fix-base": "npm run lint-base -- --fix",
     "lint-fix": "npm run lint-fix-base -- \"./**/*.{js,json,md}\"",
     "lint-styles-base": "stylelint --custom-formatter=node_modules/stylelint-formatter-pretty",
-    "lint-styles": "npm run lint-styles-base -- app/*.scss app/components/**/*.scss",
+    "lint-styles": "npm run lint-styles-base -- $npm_package_config_style_paths",
     "lint-styles-fix-base": "npm run lint-styles-base -- --fix",
-    "lint-styles-fix": "npm run lint-styles-fix-base -- app/*.scss app/components/**/*.scss",
+    "lint-styles-fix": "npm run lint-styles-fix-base -- $npm_package_config_style_paths",
     "lint-check": "eslint --print-config .eslintrc.js | eslint-config-prettier-check",
     "lint-ci": "npm run lint && npm run lint-styles && npm run flow",
     "package": "npm run build && build --publish never",
@@ -36,6 +36,9 @@
     "test-e2e": "cross-env NODE_ENV=test BABEL_DISABLE_CACHE=1 node --trace-warnings ./test/runTests.js e2e",
     "test-watch": "npm test -- --watch"
   },
+  "config": {
+    "style_paths": "app/styles/*.scss app/components/**/*.scss"
+  },
   "browserslist": "electron 2.0",
   "engines": {
     "node": ">=8.2.1",