ext/latex_mods: Fix TypeError for alt tags

When not using alt tags on figures, latex_mods.py failed with a TypeError. The
problem was, that the strings are not necessarily unicode, but the escape_map
used only works with unicodes.

Fixes #12

Signed-off-by: Erik Bernoth <erik.bernoth@gmail.com>
Merge pull request #15 from erikb85/fix-shortcaptionerror

ext/latex_mods: Fix TypeError for alt tags