Added urlview keybinding

1.  A shortcut of 'b' was used as the default binding for passing
comment body text to urlview (this is similar to the default of C-b in
the 'mutt' text email client)
2.  The `comment_urlview` SubmissionController function was added to
recieve the SUBMISSION_OPEN_IN_URLVIEWER keypress request.
3.  The `open_urlview` terminal function was added to handle the urlview
request.  It passes the comment body data to urlview via a Popen
process.
4.  A test case was added to ensure this new code path is executed
5.  Small formatting changes, mostly line length

rtv/docs.py

@@ -44,6 +44,7 @@ HELP = """
   `h` or `LEFT`       : Return to subreddit mode
   `l` or `RIGHT`      : Open the selected comment in a new window
   `SPACE`             : Fold the selected comment, or load additional comments
+  `b`                 : Display URLs with urlview
 """
 
 COMMENT_FILE = """