Allow Template.render() to be called multiple times

Allow Template.render() to be called multiple times, for the use case where a single template is rendered against multiple Contexts.

README.md

@@ -168,6 +168,8 @@ of template tags. You will need to call the `registerTag` API which accepts a
 closure to handle the parsing. You can find examples of the `now`, `if` and
 `for` tags found inside `Node.swift`.
 
+Custom template tags must be registered prior to calling `Template.render` the first time.
+
 The architecture of Stencil along with how to build advanced plugins can be found in the [architecture](ARCHITECTURE.md) document.
 
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