Allow multi-flag parameters

[akhoury6]

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. This is an unusual paradigm and provides no extra functionality over regular :flag parameters.

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:

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 -vvv
{:verbose => 3}

[akhoury6]

Just a few moments, I'll have the tests sorted out. Forgot about them.

[akhoury6]

The remaining failures seem to be unrelated, and are the same failing tests as in the other PR that I submitted.