Fix thaoe remote to work with latest tahoe-lafs (v. 1.10.0)

Hi Joey, 

I send you a small patch to Tahoe.hs which fixes "initremote repo
type=thaoe" when using the latest tahoe-lafs release available for
download.

I would like to add few things if i'm able (it's my first time for
haskell, and I'm a bit lost as now):

1) add an optional parameter or envvar for a root dir cap and switch
   from storing anonymous files to saving a tree of dirs and files
   (much like the webdav back-end) so that i can later renew the
   leases on the files to prevent tahoe's garbage collection process
   expiration (for details see
   https://tahoe-lafs.org/trac/tahoe-lafs/browser/docs/garbage-collection.rst#client-side-renewal)

   The poor man way to that without touching git-annex is to in some
   way collect the caps of the files from annex metadatas and to link
   them to a directory, which where i can then run "tahoe deep-check
   --add-lease" on.

2) add convergence as an optional parameter. As of now many of the
   files that i'm willing to manage with git-annex are already on my
   grid, mostly because i've used tahoe's backup command to upload
   them in the past. By using the same convergence value that i've
   already setup on my other tahoe client installation i would be able
   to save much time and space by avoiding duplicates.

Do you have any good pointers to pieces code of git-annex that i can
read and hack to try to implement this or any other suggestion?

let me know,

Alberto