support to only transform rules filtered by mediaQuery option

.gitignore

@@ -5,3 +5,5 @@ npm-debug.log*
 node_modules
 .idea
 postcss-px-to-em-master
+.vscode
+.history

README.md

@@ -117,7 +117,7 @@ Default Options:
     - If value is regexp, it checks to see if the selector matches the regexp.
         - `[/^body$/]` will match `body` but not `.body`
 - `minPixelValue` (Number) Set the minimum pixel value to replace.
-- `mediaQuery` (Boolean) Allow px to be converted in media queries.
+- `mediaQuery` (Boolean or Regexp or Array) Allow px to be converted in media queries.
 - `replace` (Boolean) replaces rules containing vw instead of adding fallbacks.
 - `exclude` (Array or Regexp) Ignore some files like 'node_modules'
     - If value is regexp, will ignore the matches files.

index.js

@@ -149,5 +149,11 @@ function declarationExists(decls, prop, value) {
 }
 
 function validateParams(params, mediaQuery) {
+  if (Object.prototype.toString.call(mediaQuery) === '[object RegExp]') {
+    return mediaQuery.test(params)
+  } else if (Object.prototype.toString.call(mediaQuery) === '[object Array]') {
+    return mediaQuery.some(mq => mq.test(params))
+  }
+
   return !params || (params && mediaQuery);
 }

spec/px-to-viewport.spec.js

@@ -221,6 +221,16 @@ describe('mediaQuery', function () {
 
     expect(processed).toBe(expected);
   });
+
+  it('should only replace px inside media queries if opts.mediaQuery is a RegExp or RegExp array', function() {
+    var options = {
+      mediaQuery: /max-width:\s*750px/
+    };
+    var processed = postcss(pxToViewport(options)).process('h1 { font-size: 32px } @media (max-width: 750px) { .rule { font-size: 16px } } @media (min-width: 751px) { .rule { font-size: 18px } }').css;
+    var expected = 'h1 { font-size: 32px } @media (max-width: 750px) { .rule { font-size: 5vw } } @media (min-width: 751px) { .rule { font-size: 18px } }';
+
+    expect(processed).toBe(expected);
+  });
 
   it('should replace px inside media queries if it has params orientation landscape and landscape option', function() {
     var options = {