Trigger BufWritePost event via BufWriteCmd/FileWriteCmd

Closes jamessan/vim-gnupg#23
Signed-off-by: James McCoy <vega.james@gmail.com>
Emit FileWritePre/Post events when handling FileWriteCmd

Signed-off-by: James McCoy <vega.james@gmail.com>
Don't change modified state of buffer for FileWriteCmd

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