Allow definition of actions specific to GnuPG-managed buffers

If there are specific actions that should take place when editing a
GnuPG-managed buffer, an autocmd for the User event and GnuPG pattern
can be defined.  For example, the following will set 'textwidth' to 72
for all GnuPG-encrypted buffers:

    autocmd User GnuPG setl textwidth=72

This will be triggered before any BufRead or BufNewFile autocmds, and
therefore will not take precedence over settings specific to any
filetype that may get set.

Closes jamessan/vim-gnupg#10

Signed-off-by: James McCoy <vega.james@gmail.com>