Add command-line option -S/--severity

Specifies the maximum severity of errors to handle.  For example,
specifying "-S warning" means that errors of severity "info" and
"style" are ignored.

Signed-off-by: Martin Schwenke <martin@meltin.net>

src/ShellCheck/Interface.hs

@@ -35,7 +35,8 @@ data CheckSpec = CheckSpec {
     csScript :: String,
     csCheckSourced :: Bool,
     csExcludedWarnings :: [Integer],
-    csShellTypeOverride :: Maybe Shell
+    csShellTypeOverride :: Maybe Shell,
+    csMaxSeverity :: Severity
 } deriving (Show, Eq)
 
 data CheckResult = CheckResult {
@@ -49,7 +50,8 @@ emptyCheckSpec = CheckSpec {
     csScript = "",
     csCheckSourced = False,
     csExcludedWarnings = [],
-    csShellTypeOverride = Nothing
+    csShellTypeOverride = Nothing,
+    csMaxSeverity = StyleC
 }
 
 newParseSpec :: ParseSpec