Adds --full-text option to direct-messages command. Fixes #371

[porras]

What title says. This option is supported directly by the twitter gem and API so it's a very small change.

    bundle exec bin/t help direct_messages
    Usage:
      t direct_messages

    Options:
      -c, [--csv], [--no-csv]                        # Output in CSV format.
      -d, [--decode-uris], [--no-decode-uris]        # Decodes t.co URLs into their original form.
      -l, [--long], [--no-long]                      # Output in long format.
=>    -f, [--full-text], [--no-full-text]            # Fetch full texts.                <=
      -n, [--number=N]                               # Limit the number of results.
                                                     # Default: 20
      -a, [--relative-dates], [--no-relative-dates]  # Show relative dates.
      -r, [--reverse], [--no-reverse]                # Reverse the order of the sort.
      -C, [--color=COLOR]                            # Control how color is used in output
                                                     # Default: auto
                                                     # Possible values: icon, auto, never
      -P, [--profile=FILE]                           # Path to RC file
                                                     # Default: /Users/sergio/.trc

    Returns the 20 most recent Direct Messages sent to you.

I guess it would make sense to make this the default when --csv is present too, but I didn't want to change the behaviour of a preexisting option unless maintainers suggest so (feel free to suggest this or any other change).

Thanks for the t tool, it's awesome!