Issue 837 flag to include only certain warnings

Issue https://github.com/koalaman/shellcheck/issues/837

Add an --include option, which creates a whitelist of warnings to report
on, the opposite of --exclude.

src/ShellCheck/Checker.hs

@@ -94,11 +94,13 @@ checkScript sys spec = do
             (parseMessages ++ map translator analysisMessages)
 
     shouldInclude pc =
-        let code     = cCode (pcComment pc)
+            severity <= csMinSeverity spec &&
+            case csIncludedWarnings spec of
+                Nothing -> code `notElem` csExcludedWarnings spec
+                Just includedWarnings -> code `elem` includedWarnings
+        where
+            code     = cCode (pcComment pc)
             severity = cSeverity (pcComment pc)
-        in
-            code `notElem` csExcludedWarnings spec &&
-            severity <= csMinSeverity spec
 
     sortMessages = sortBy (comparing order)
     order pc =
@@ -137,6 +139,13 @@ checkRecursive includes src =
         csCheckSourced = True
     }
 
+checkOptionIncludes includes src =
+    checkWithSpec [] emptyCheckSpec {
+        csScript = src,
+        csIncludedWarnings = includes,
+        csCheckSourced = True
+    }
+
 prop_findsParseIssue = check "echo \"$12\"" == [1037]
 
 prop_commentDisablesParseIssue1 =
@@ -274,6 +283,21 @@ prop_sourcedFileUsesOriginalShellExtension = result == [2079]
         csCheckSourced = True
     }
 
+prop_optionIncludes1 =
+    -- expect 2086, but not included, so not reported
+    null $ checkOptionIncludes (Just [2080]) "#!/bin/sh\n var='a b'\n echo $var"
+
+prop_optionIncludes2 =
+    -- expect 2086, included, so its reported
+    [2086] == checkOptionIncludes (Just [2086]) "#!/bin/sh\n var='a b'\n echo $var"
+
+prop_optionIncludes3 =
+    -- expect 2086, no inclusions provided, so its reported
+    [2086] == checkOptionIncludes Nothing "#!/bin/sh\n var='a b'\n echo $var"
+
+prop_optionIncludes4 =
+    -- expect 2086 & 2154, only 2154 included, so only its reported
+    [2154] == checkOptionIncludes (Just [2154]) "#!/bin/sh\n var='a b'\n echo $var\n echo $bar"
 
 return []
 runTests = $quickCheckAll