Add support for pdftools midnight mode

[Merlin]

pdftools has a midnight mode where pdfs are displayed with light text on dark background. The default theme looks odd in combination with the moe theme. Maybe this could be an easy addition to the theme.

The colors are configured through the variable pdf-view-midnight-colors:
https://github.com/politza/pdf-tools/blob/master/lisp/pdf-view.el#L111