afvr[?] manipulate register based arguments

• afvr list register based arguments
• afvr* same as afvr but in r2 commands
• afvr [reg] [name] ([type]) define register arguments
• afvrj return list of register arguments in JSON format
• afvr- [name] delete register arguments at the given index
• afvrg [reg] [addr] define argument get reference
• afvrs [reg] [addr] define argument set reference