Simplify VikiLinkBot::Input implementation

[ ] Remove VLisp-specific lexing. It is now expected that VLisp strings are always quoted.
[ ] Replace String#scan with an implementation based on StringScanner.
[ ] Implement a class method for a quick lookup of the command name (without "full" parsing of the line).

StringScanner will allow to distinguish between command name, arguments and wikis at lexing time.

thilp / vlb-rb