Make live-preview hook and kill-buffer-hooks buffer-local
Take global hooks and make them buffer-local.
markdown-live-preview-remove-on-kill applies to both the Markdown buffers and
the buffers used to preview the export, so it needs to be re-added in two
places, where the export-viewing buffer is created, and when markdown-mode is
turned on.
The test test-markdown-ext/live-preview-exports adequately covers whether
these hooks run.
[jrblevin@sdf.org: Squashed two commits and rebased.
Added GH-63 PR text to commit message for reference.]
Signed-off-by: Jason Blevins <jrblevin@sdf.org>