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

Implemented implicit keep to execute when not canceled or when the preceeding action execution fails.

Properly implemented the code dumper. Dumps are now printed in a stream. The individual opcode and operand implementations no longer use printf()s.

Exported sieve-code-dumper from sieve-interpreter containing all code dumping related implementation. Now to remove all printfs....

Grouped runtime parameters into a single runtime environment and started implementation of result composition/execution.

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.

* Started the lda plugin source

* Added plugins as static libraries (for now)

* Fixed a bug in the require command generation.