Add a 'list' method to the command line client that lists post ids

...and urls, optionally filtered by tag or post type.

Here's what the output looks like:

> $ tumblr help list
> Usage:
>  tumblr list
>
> Options:
>  [--tag=TAG]                  # Post tag
>  [--type=TYPE]                # Post type: text, quote, link, answer, video, audio, photo, chat
>  [--credentials=CREDENTIALS]  # The file path where your Tumblr OAuth keys are stored. Defaults to ~/.tumblr.
>   [--host=HOST]                # The hostname of the blog you want to post to i.e. "YOUR-NAME.tumblr.com"
>
> List Tumblr post_ids and URLs for posts, optionally with a particular tag or post type


Or if you actually run the command you'll get something like this (provided you've got the correct authorization, etc):

> $ tumblr list --type=quote
> 39948369647 => http://cca.elstudio.us/post/39948369647/please-just-do-this-if-you-do-nothing-else-git
> 39703564743 => http://cca.elstudio.us/post/39703564743/think-about-what-other-features-are-missing-in-the

Hope you'll find it helpful!