Option `cwd` not handled well and pdfs in subdirs not rendered

[tbung]

Currently, when the option cwd is set, the items shown are correctly listed
from that directory, however the rendering script is not told about this and
still tries to find files relative to the working directory of vim.

Also, when pdf files (and I assume video files) are in subdirectories, the
conversion to png does not work because pdftoppm is given the base name
instead of the path to the file relative to the current directory.

I have fixed both issues locally and would be happy to submit a pr if you are
interested.

[elianiva]

PR is more than welcome :)

[siduck]

@elianiva #12