add @CustomElement

CHANGELOG.md

+#### 0.10.4
+  * Added `CustomElement` annotation. This can be added to any class to register
+    it with a tag in the main document.
+  * Added a `web_components.dart` file which exports all the annotations
+    provided by this package. Note that in later breaking releases
+    `html_import_annotation.dart` and `custom_element_proxy.dart` will likely
+    move into the `src` folder, so switching to the `web_components.dart` import
+    is recommended.
+
 #### 0.10.3
   * Added `generateWebComponentsBootstrap` method to the main `web_components`
     transformer file which accepts a `Transform` and a `Resolver`. You can use
     this function from any transformer and share the resolver you already have.
   * Fixed up the bootstrap call in `ScriptCompactor` to not use `=>` syntax
     since it has a declared return type of `void`. This could previously cause
     a checked mode error if the original program returned something from `main`.
 
 #### 0.10.2+1
   * Minor cleanup to changelog and tests.
@@ skipping 116 matching lines @@
   * Reverts back to what we had in 0.3.4. (The platform.js updates in 0.3.5 had
     breaking changes so we are republishing it in 0.4.0)
 
 #### 0.3.5
   * Added `registerDartType` to register a Dart API for a custom-element written
     in Javascript.
   * Updated to platform 0.3.3-29065bc
 
 #### 0.3.4
   * Updated to platform 0.2.4 (see lib/build.log for details)