Allow opening braces in front of a tag

As opening braces were allowed as the first symbol in the tag content,
the tag detection was greedy. It also removed single braces placed in
front of the real tag.
Thus no opening braces could be placed directly before a replaced text.

The problem occured in my case when using the function template of pdv (https://github.com/tobyS/pdv/blob/master/templates_snip/function.tpl), where this is used for the param part of the doc block: ```${{{?func: vmustache#IncrementCounter("params")}}:```. The result there needs to be "${2:" for the UltiSnip completion, without this change it is "$2:"