fix: Prevent duplicate style injection (#68)

* fix: Cache style ids on injector function to prevent duplicate injections

* fix: Add postcss as peer dependency

Author: znck

package.json

@@ -50,6 +50,7 @@
     "vue-template-compiler": "^2.5.16"
   },
   "peerDependencies": {
+    "postcss": ">=6.0",
     "vue-template-compiler": "*"
   },
   "dependencies": {

src/assembler.ts

@@ -77,7 +77,7 @@ export function assembleFromSource(
   // language=JavaScript
   const inlineCreateInjector = `function __vue_create_injector__() {
   const head = document.head || document.getElementsByTagName('head')[0]
-  const styles = {}
+  const styles = __vue_create_injector__.styles || (__vue_create_injector__.styles = {})
   const isOldIE =
     typeof navigator !== 'undefined' &&
     /msie [6-9]\\\\b/.test(navigator.userAgent.toLowerCase())