Thousand separator for Printf -- aka : pretty print financial values

[nmingotti]

• printf has a nice feature that let us print big numbers as "1,234.12". We make this available also Cuis printf. 
• in printf the modifier is $' , i don't want to use that in Cuis because it is our one and only literal string delimiter, so i use $_ as modifier i.e. %d thousand separated is %_d, and %0.2f thousand separated would be %_0.2f
• I use _ as default thousand separator in agreement with what Perl, Ruby and recently also Python accept. 
• The user has possibility of deciding his own thousand and decimal separator. Indeed in Italy for example we would print financial values as e.g. 1,234.12 . 
• new tests are added
• new tests are kept separated from the old ones, they are progressively numbered
• Detected bug in existing Printf code, made test to keep it in sight and remember it is there. It is an ugly bug on printing big values. see test101_X .