Speed up file addition

Improve fb2 parser
Fix extension checking

TODO

@@ -77,6 +77,8 @@ TODO
         ✓ mobi, azw support
             Limit the extra files produced by KindleUnpack
                 Have them save to memory
+        ✓ fb2 support
+            Images need to show up in their placeholders
     Other:
         ✓ Define every widget in code
     Bugs:
@@ -84,7 +86,6 @@ TODO
         Clean up 'switch' page layout
         Colors aren't loaded properly for annotation previews
         Cover page shouldn't be scolled midway
-        It's possible the addition function is also parsing the whole book.
 
     Secondary:
         Graphical themes
@@ -98,7 +99,7 @@ TODO
         Use embedded fonts + CSS
         Scrolling: Smooth / By Line
         Shift to logging instead of print statements
-        txt, doc, chm, djvu, fb2 support
+        txt, doc, chm, djvu support
         Include icons for filetype emblems
         Comic view modes
             Continuous paging