Add Scan and Help commands

[mrbiggred]

Spawned from #23. Currently the CLI does not command line parsing and just assume the first argument passed in is the repository. We need to create a framework/design for parsing more complicated command lines.

This issue only encompasses adding the framework and adding the scan and help commands. No other commands should be added.

Currently the CLI supports the following command which scans the repository:

freshli-cli http://url/to/git/repo

After this issue is implemented the above existing command should still work but with the scan command:

freshli-cli scan <git-repo>

There should also be a help command:

freshli-cli help [command]

The help should display something like the below. Will have to work with someone smarter then myself to create a nice looking help message.

Freshli checks your projects dependency Freshness (i.e. how out of up to date are your dependencies). 

Supported commands:
  scan: Scan the given repository for freshness history.
    freshli-cli scan <git-repo>
    
  help: Display the help for commands.  If not arguments are supplies will show all the high level commands (i.e. what you are viewing now).
    freshli-clie help [command]

Examples:

Scan the Freshli Ruby test repository:
  freshli-cli scan https://github.com/corgibytes/freshli-fixture-ruby-nokotest

Help for the scan command:
  freshli-cli help scan

The help command for scan:

freshli-cli help scan

Should display something like:

Computes the historical Freshness of your repository.  Data is returned in <some format>.

scan: freshli-cli scan <git-repo>

  Arguments:
    git-repo: The Git repository to parse.  Supports `git://` or `https://` paths.

Examples:

Scan the Freshli Ruby test repository:
  freshli-cli scan https://github.com/corgibytes/freshli-fixture-ruby-nokotest