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>
Restore the :silent command when encrypting the buffer

This was lost in the refactor of external command handling, commit
7d102c843, and was causing an unnecessary "Press ENTER …" prompt to be
emitted whenever the buffer was written out.

Signed-off-by: James McCoy <vega.james@gmail.com>
Use filereadable() instead of glob() to detect whether a file exists.

glob() reports false positives if the filename has metacharacters in it
(e.g., "[...]"), which causes Vim to incorrectly open a new, blank
buffer instead of the specified file.

Closes jamessan/vim-gnupg#12

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