Auto-create directory structure for output file

[AMeng]

If you specify an output file to a directory that does not exist, a Ruby error is raised.

$ chefspec-bootstrap recipe.rb -o a/b/c/d.txt
/chefspec-bootstrap/lib/chefspec-bootstrap.rb:61:in `initialize': No such file or directory - a/b/c/d.txt (Errno::ENOENT)
    from /chefspec-bootstrap/lib/chefspec-bootstrap.rb:61:in `open'
    from /chefspec-bootstrap/lib/chefspec-bootstrap.rb:61:in `generate'
    from /chefspec-bootstrap/bin/chefspec-bootstrap:35:in `<top (required)>'
    from /.rvm/gems/ruby-1.9.3-p429/bin/chefspec-bootstrap:19:in `load'
    from /.rvm/gems/ruby-1.9.3-p429/bin/chefspec-bootstrap:19:in `<main>'
    from /.rvm/gems/ruby-1.9.3-p429/bin/ruby_noexec_wrapper:14:in `eval'
    from /.rvm/gems/ruby-1.9.3-p429/bin/ruby_noexec_wrapper:14:in `<main>'

The directories should be auto-created if they do not exist.

[AMeng]

Fixed in #17