fix #1102. After the fix, to escape the brackets in utterances, just use { or }. List some escape examples as below.

• this is another {@from = one} from {@to = tokyo} \in japan => this is another {@from = one} from {@to = tokyo} \in japan
• this is another \{@@from = one} from {@@to = tokyo} => this is another \one from tokyo (entity names are @from and @to)

Currently we only support escape three chars: { or } or .