Configurable "sent" folder name and dynamic contacts export

[ghost]

One can now set the "Sent" mailbox name which enables multi lingual support for Gmail as well as for own server. Contacts are now extracted out of the configured sent folder.
Other configurable folder names will follow :)

[danchoi]

Thank you. I'll take a look this weekend.

[danchoi]

@mintao would you like to help me maintain this project? I would be happy to give you admin rights and let you merge things directly. Thanks for the patch!

[notalex]

@mintao If the purpose of this PR is to display sent for non english Gmail accounts when viewing the mailbox list, then I do not think it is on the right track. I did not work for me when I tried it for my Hindi gmail account. This experimental branch might be what we should be working upon. It needs to be able to correctly compare the utf-8 encoded mailbox names.

[notalex]

@mintao The experimental_branch can now alias all Gmail folder names(labels) for non English languages. I will be merging it into master soon. Let me know if this is what you need, so I that can close this PR.

[notalex]

@mintao I understand now that this patch is specifically meant to address the issue #102 (unable to generate contacts file for non English accounts). Your code will require minor modifications to it to make it work with the multi_lingual patch which already deals with another issue related to non English mailboxes. Let me know if you are up for it and I will guide you through the modifications.

[notalex]

@mintao I have fixed the issue with contacts generation and other reported problems with non English Gmail accounts in version 2.9.0. Some additional configuration will be required as documented in this wiki.