Added validat_context method to command arguments for the to-be-implemented argument context validation.

Changed argument generator function prototype to assign responsibility of advancing to the next argument to the generator itself.

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.

Implemented support for additional parameters to match-types and implemented validation for the relational extension.

Created skeletons for regex and relational extensions. These are to be developed simultaneously with the match-type support in general.