Trigger BufWritePost event via BufWriteCmd/FileWriteCmd

> Is writing partial files supported by vim-gnupg?

Yes, although not as well as it should be. I've started working on a fix for that and will incorporate these changes too.

> What does the `'[, ']` range refer to in the `w` ex command being used?

Vim sets `'[` and `']` when writing to a file, defaulting to `℅`.  For FileWriteCmd, the range will be a subset of the entire buffer.