Add json1 format that ignores tabs

The new json1 format works just like json except that it treats tabs as
single characters instead of 8-character tabstops.

The main use case is to allow editors to pass -fjson1 so that they can
consume the json output in a character-oriented way without breaking
backwards compatibility.

Also addresses #1048.

shellcheck.1.md

@@ -153,7 +153,7 @@ not warn at all, as `ksh` supports decimals in arithmetic contexts.
 
 :   Json is a popular serialization format that is more suitable for web
     applications. ShellCheck's json is compact and contains only the bare
-    minimum.
+    minimum.  Tabs are 8 characters.
 
         [
           {
@@ -167,6 +167,11 @@ not warn at all, as `ksh` supports decimals in arithmetic contexts.
           ...
         ]
 
+**json1**
+
+:   This is the same as shellcheck's json format, but tabs are treated as
+    single characters instead of 8-character tabstops.
+
 *quiet*
 
 :   Suppress all normal output. Exit with zero if no issues are found,