Add filters with symbols as name

[k94ll13nn3]

Add possibility to define filters whose name is only composed of symbols (like =, !, >, ...)

Need to define an operator (with \0 as indicator) and the parsing for the symbols.

Example: {tag:(operator)&+value} or {tag:(operator)**+value}

[k94ll13nn3]

Closing this in favor of #51. Won't allow the creation of filters with symbols as name, but only the addition of an alternative name to a filter.