Add file handler extensions for crt, key, pem, pub

Handy to be able to open public and private keys on the chromebook for copying, pasting, and concatenating.

manifest.json

@@ -55,6 +55,7 @@
         "coffee",
         "conf",
         "cpp",
+        "crt",
         "cs",
         "csh",
         "css",
@@ -89,6 +90,7 @@
         "js",
         "json",
         "jsx",
+        "key",
         "ksh",
         "less",
         "lisp",
@@ -110,6 +112,7 @@
         "micro",
         "obc",
         "patch",
+        "pem",
         "php",
         "pkb",
         "pkg",
@@ -118,6 +121,7 @@
         "pls",
         "pm",
         "ps",
+        "pub",
         "py",
         "r",
         "rake",
@@ -159,4 +163,4 @@
       ]
     }
   }
-}
+}