Added implementation for url_file_attachment option.

Using url_file_attachment user can specify if he like to change objects
like 'file_share' marked as external (in Slack servers point of view)
to attachments, so the only value would be remembered URL for the
"uploaded" document as an attachment. Or, treat it as is, and let the
slack-backup to produce file which contain the URLs and corresponding
local file names for such files.

README.rst

@@ -90,6 +90,28 @@ where:
   created, but you'll (obviously) lost all the records. Besides the db file,
   assets directory might be created for downloadable items.
 
+There is one more switch to take into consideration -
+``-f/--url_file_to_attachment`` which influence the way how external file
+share would be treated. First of all, what is *external* file share from slack
+point of view, one could ask. Slack have some sort of integration with Goolgle
+services, like Googla Drive, which provide slack users to create or "upload"
+files from Google Drive. "Upload", since no uploading actually takes place,
+and only URL is provided for such "uploads". By default `slack-backup` will
+create a file which is prefixed ``manual_download_`` which will contain url and
+destination path to the file, where user should manual download file to.
+Example file contents:
+
+.. code::
+
+   http://foo.bar.com/some/file --> assets/files/83340cbe-fee2-4d2e-bdb1-cace9c82e6d4
+   http://foo.bar.com/some/other/file --> assets/files/8a4c873c-1864-4f1b-b515-bbef119f33a3
+   http://docs/google.com/some/gdoc/file --> assets/files/ec8752bc-0bf8-4743-a8bd-9756107ab386
+
+By setting ``url_file_to_attachment`` flag (or making it set to true in config
+file) such "uploads" would be internally converted into Slack "attachment",
+which internally is an object to store external links, so there is no need for
+user interaction.
+
 During DB creation, all available messages are stored in the database. On the
 next run, ``fetch`` would only take those records, which are older from
 currently oldest in DB. So that it will only fetch a subset of the overall of
@@ -136,6 +158,7 @@ For convenience, you can place all of needed options into configuration file
    theme = plain
 
    [fetch]
+   url_file_to_attachment = false
    user =
    password =
    team =