fix(mv): add -n option, make -f default behavior

[nfischer]

This lets mv() have -f as the default behavior (the flag exists for legacy reasons). I introduce the -n flag, which will allow the opposite behavior. This is the default behavior on Bash.

This also introduces a modification to parseOptions(), allowing dictionaries to be specified of the following form:

options = common.parseOptions(options, {
  'f': '!no_force', // the boolean opposite of "no_force"
  'n': 'no_force'
});

The empty string will still cause no_force to default to false, as it normally would. Options prefixed with ! have the special meaning that the appearance of that commandline flag sets the option to false instead of true. So passing the -f flag will cause options.no_force to be false.

If someone passes in a flag like -fn, this will set options.no_force = true, taking whichever is last, which is consistent with most core-utils. -nf will likewise set options.no_force = false.

[nfischer]

This is related to #210 (although this is the mv equivalent), and the discussion in #325. If #325 is refactored to change default behavior, then this blocks #325.

[ariporad]

LGTM!