Allow multi-flag parameters

Some tools need to allow the repeated input of certain valueless parameters. One such example is the use of `v` for `verbose`, where the addition of more v's increases the verbosity of the output. This behavior can be seen in tools such as curl and ssh.

Prior to this change, when the `:multi` option was passed in to a `:flag` or `:boolean` parameter, the parser would return either a a `true` value or an empty array depending on if the flag was passed in the command line or not.

After this change, the `:flag` with `:multi` set will instead count the number of times the flag was passed. Flag options without `:multi` set still exhibit the original behavior of simple `true`/`false` values.

Example config:
```ruby
opts = Optimist::options do
    opt :verbose, "Enables verbose output. Twice will enable very-verbose output", multi: true
end
```

Before change:
```
$ mytool
{:verbose => []}

$ mytool -vvv
{:verbose => true}
```

After change:
```
$ mytool
{:verbose => 0}

$ mytool -vv
{:verbose => 3}
```

lib/optimist.rb

@@ -245,7 +245,8 @@ def parse(cmdline = ARGV)
     @specs.each do |sym, opts|
       required[sym] = true if opts.required?
       vals[sym] = opts.default
-      vals[sym] = [] if opts.multi && !opts.default # multi arguments default to [], not nil
+      vals[sym] = [] if opts.multi && !opts.default && !opts.flag? # multi arguments default to [], not nil
+      vals[sym] = 0 if opts.multi && !opts.default && opts.flag? # multi argument flags default to 0 because they return a count
     end
 
     resolve_default_short_options!
@@ -283,6 +284,11 @@ def parse(cmdline = ARGV)
       # The block returns the number of parameters taken.
       num_params_taken = 0
 
+      if @specs[sym].multi? && @specs[sym].flag?
+        given_args[sym][:params][0] ||= 0
+        given_args[sym][:params][0] += 1
+      end
+
       unless params.empty?
         if @specs[sym].single_arg?
           given_args[sym][:params] << params[0, 1]  # take the first parameter
@@ -765,6 +771,7 @@ def initialize
   end
   def flag? ; true ; end
   def parse(_paramlist, neg_given)
+    return _paramlist[0] if @multi_given
     return(self.name.to_s =~ /^no_/ ? neg_given : !neg_given)
   end
 end