Export metadatas

Add an option `--export` to export the metadatas (workitems, reviewers,...) in the commit message
(like they were before checkin in tfs) : 

    git-tfs-work-item: 1245 associate
    git-tfs-code-reviewer: paul
    git-tfs-security-reviewer: arthur
    git-tfs-performance-reviewer: john

Just have to use the command :
`git filter-branch -f --msg-filter 'sed "s/^git-tfs-id:.*$//g"' -- --all`

and you will be able to import back in another tfs or somewhere else ;)

PS : if there is branches, merge commit will never be checkinable in another tfs server