Improve tool ui

Small fixes to improve the user-interface of tools.

* nit_dir is more robust and display errors if invalid
* option --nit-dir is really needed
* remove useless and buggy --clib-path and NIT_CC_PATH
* nit does not collect options after the program file but let as options for the interpreter
* 3 -v to show class-level information, 4 -v to show property-level information
It is not very clear to me what this attribute really does because I thought that `ToolContext` stops to look for options at first non-option argument (like most implementations of `getopt` do).