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.

slack_backup/reporters.py

@@ -403,7 +403,7 @@ class StaticHtmlReporter(Reporter):
                 'nick': msg.user.name}
 
         link = '<a href="{url}">{title}</a>'
-        attachement_msg = []
+        attachment_msg = []
 
         if msg.attachments:
             for att in msg.attachments:
@@ -429,9 +429,9 @@ class StaticHtmlReporter(Reporter):
                                                     link.format(**match))
                     else:
                         att_text = att.fallback
-                attachement_msg.append(att_text)
+                attachment_msg.append(att_text)
 
-        data['msg'] += '<br>'.join(attachement_msg)
+        data['msg'] += '<br>'.join(attachment_msg)
         return data