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template engine
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* [improve] refactoring
* [change] variable and tag can't be empty
* [change] ``template.code`` change to ``template.code_builder``
* [change] ``Template.__init__(self, text, context=None, ...)``
change to ``Template.__init__(self, text, default_context=None, ...)``
* [change] ``Template.TOKEN_EXPR_START`` change to ``Template.TOKEN_VARIABLE_START``
* [change] ``Template.TOKEN_EXPR_END`` change to ``Template.TOKEN_VARIABLE_END``