Improved error message for unknown Sieve extension to account for core commands included as an extension.

Implemented support for overriding default argument implementations of number, string and string-list.

Changed validator's command syntax validation such that command implementations don't have to call the argument, test and block validation functions explicitly.

Changed argument validation to record the first positional argument into the command context by default. Also furter improved validator's error handling.

* Fixed a bug in the require command generation.