chore: make prettier dependency optional

closes #84

Author: haoqunjiang

Diff for: lib/compileTemplate.ts

@@ -176,6 +176,12 @@ function actuallyCompile(
             parser: 'babel'
           })
         } catch (e) {
+          if (e.code === 'MODULE_NOT_FOUND') {
+            tips.push(
+              'The `prettify` option is on, but the dependency `prettier` is not found.\n' +
+                'Please either turn off `prettify` or manually install `prettier`.'
+            )
+          }
           tips.push(
             `Failed to prettify component ${options.filename} template source after compilation.`
           )

Diff for: package.json

@@ -62,8 +62,10 @@
     "merge-source-map": "^1.1.0",
     "postcss": "^7.0.14",
     "postcss-selector-parser": "^6.0.2",
-    "prettier": "^1.18.2",
     "source-map": "~0.6.1",
     "vue-template-es2015-compiler": "^1.9.0"
+  },
+  "optionalDependencies": {
+    "prettier": "^1.18.2"
   }
 }