Separate normalizers out, fix config flag checks

src/normalizers.js

@@ -0,0 +1,32 @@
+import fs from 'fs';
+import path from 'path';
+
+export function boolean(value) {
+    if (typeof value === 'boolean') {
+        return value;
+    } else if (value === 'true') {
+        return true;
+    } else if (value === 'false') {
+        return false;
+    }
+
+    throw 'Must be true or false.';
+}
+
+export function filePath(value) {
+    const unwrapped = value.replace(/"(.*?)"/, '$1');
+    const absolute = path.resolve(unwrapped);
+
+    let fileExists;
+    try {
+        fileExists = fs.existsSync(absolute) && fs.statSync(absolute).isFile();
+    } catch (ex) {
+        fileExists = false;
+    }
+
+    if (fileExists) {
+        return absolute;
+    } else {
+        throw 'File not found at ' + absolute + '.';
+    }
+}