optional args support

[nanobowers]

Would be nice to support an opt that can be given with/without arguments. Without arguments (or defaults), it would take on flag-like behavior.

opt :abc, "flag-or-str", :type => :stringflag

cmd             # opts[:log] == nil
cmd --abc       # opts[:log] == true
cmd --abc hello # opts[:log] == "hello"

One such case would be a "log" option that couldtake a default value (output.log) if enabled without an argument, but could have that value overridden
examples:

opt :log, "maybe-a-log", :type => :stringflag, :default => "output.log"

cmd                   # opts[:log] == nil
cmd --log             # opts[:log] == "output.log"
cmd --log another.log # opts[:log] == "another.log"

currently all options in Optimist are either flags (taking no args) or are of a type that take args. Supporting this feature may be difficult w/ the current codebase especially when taking into account how defaults should be handled, if no- args should be handled, what to do with multi and/or array types.