Quickly convert Ruby blocks between {} and begin/end.

Trigger the conversion with :B or <leader>B.

From http://eigenclass.org/hiki/Ruby+block+conversion+macros+for+Vim

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When committing to this repo, make sure the author date matches the
upstream release date.  For 0.1 released on 2005-11-14:

    $ GIT_AUTHOR_DATE=2005-11-14T00:00:00 git commit
    $ GIT_COMMITTER_DATE=2005-11-14T00:00:00 git tag -a 0.1