Improve crontab filetype detection (zyedidia#3222)

Support crontab filetype detection in the case crontab is opened via
sudoedit. Also apparently this fixes crontab filetype detection when
it is opened normally via `crontab -e` but in MacOS.

Fixes zyedidia#3172

Author: dmaluka

runtime/syntax/crontab.yaml

@@ -1,7 +1,7 @@
 filetype: crontab
 
 detect:
-    filename: "crontab$"
+    filename: "crontab$|/tmp/crontab\\.\\w+$"
     header: "^#.*?/etc/crontab"
 
 rules: