Fail on invalid eol setting

Author: sds

config/default.yml

@@ -632,7 +632,7 @@ PreCommit:
   LineEndings:
     description: 'Check line endings'
     enabled: false
-    eol: "\n"
+    eol: "\n" # or "\r\n" for Windows-style newlines
 
   XmlLint:
     enabled: false

lib/overcommit/hook/pre_commit/line_endings.rb

@@ -45,6 +45,8 @@ def eol
                   'lf'
                 when "\r\n"
                   'crlf'
+                else
+                  raise 'Invalid `eol` option specified: must be "\n" or "\r\n"'
                 end
     end
   end