os.chdir was getting confusing and hurting the log output, and
potentially the cause of a couple bugs left to fix, so this removes it,
but it means we need to pass around the pages/ absolute path into the
markdown parser, because it relies on knowing both the absolute path
now (to open files), and also the path relative to the pages dir in
order to know where to stop reading parent files/how to generate proper
URL-like references to other files.
probably this should be refactored at some point to inherit the pages/
path from the SSG somehow, rather than passing it through a bunch of
methods, but this seems to work for now
fixes#22
Signed-off-by: Brian S. Stephan <bss@incorporeal.org>