Revert "Relax --abbreviations rules so that a period isn't required."

This reverts commit e461b7dd45.

Ill-advised change.  This doesn't work because we parse
strings in chunks.

MANUAL.txt

@@ -691,9 +691,9 @@ header when requesting a document from a URL:
     system default, use
     `pandoc --print-default-data-file=abbreviations`.  The only
     use pandoc makes of this list is in the Markdown reader.
-    Strings found in this list will be followed by a nonbreaking
-    space, and the period will not produce sentence-ending space
-    in formats like LaTeX.  The strings may not contain spaces.
+    Strings ending in a period that are found in this list will
+    be followed by a nonbreaking space, so that the period will
+    not produce sentence-ending space in formats like LaTeX.
 
 [`pandocfilters`]: https://github.com/jgm/pandocfilters
 [PHP]: https://github.com/vinai/pandocfilters-php

src/Text/Pandoc/Readers/Markdown.hs

@@ -1725,7 +1725,7 @@ str = do
   updateLastStrPos
   (do guardEnabled Ext_smart
       abbrevs <- getOption readerAbbreviations
-      if result `Set.member` abbrevs
+      if not (T.null result) && T.last result == '.' && result `Set.member` abbrevs
          then try (do ils <- whitespace
                       notFollowedBy (() <$ cite <|> () <$ note)
                       -- ?? lookAhead alphaNum