MINOR: args: implement a new arg type for regex : ARGT_REG

MINOR: args: implement a new arg type for regex : ARGT_REG

This one will be used when a regex is expected. It is automatically
resolved after the parsing and compiled into a regex. Some optional
flags are supported in the type-specific flags that should be set by
the optional arg checker. One is used during the regex compilation :
ARGF_REG_ICASE to ignore case.