define a FLG_COST for new option --cost

before we run out of available flags (again).
I intend to come up with a syntax that alows to specify
min/max values or ranges for multiple tunable costs with
a single option (and thus, a single flag).
Since adjusting all existing formats to the new format definition
will take time, I want to avoid spending all this effort without
any assurance that we ran out of flags once I am ready to create
a pull request.