Do not enter password of transifex at command line by security reason

Any command line options could be accessed by other users.
`tx init` command has interaction interface.
It should be used by all users by default.

doc/intl.rst

@@ -202,10 +202,14 @@ easy to fetch and push translations.
 
    .. code-block:: bash
 
-      $ tx init --user=<transifex-username> --pass=<transifex-password>
+      $ tx init
+      --user=<transifex-username> --pass=<transifex-password>
       Creating .tx folder...
       Transifex instance [https://www.transifex.com]:
       ...
+      Please enter your transifex username: <transifex-username>
+      Password: <transifex-password>
+      ...
       Done.
 
 #. Upload pot files to transifex service