MINOR: args: add type-specific flags for each arg in a list

MINOR: args: add type-specific flags for each arg in a list

These flags are meant to be used by arg checkers to pass out-of-band
information related to some args. A typical use is to indicate how a
regex is expected to be compiled/matched based on other arguments.
These flags are initialized to zero by default and it is up to the args
checkers to set them if needed.