Detect use of 'initial', 'unset', and 'revert' keywords

Fixes anandthakker#56

As IE11 does not support 'initial' and 'unset', and 'revert' is not
yet supported by pretty much any browser, detecting their usage is
valuable to developers and would allow linters to warn about it.

data/features.js

@@ -391,5 +391,17 @@ module.exports = {
   'css-image-set': {
     properties: [''],
     values: ['image-set']
+  },
+  'css-initial-value': {
+    properties: [''],
+    values: ['initial']
+  },
+  'css-unset-value': {
+    properties: [''],
+    values: ['unset']
+  },
+  'css-revert-value': {
+    properties: [''],
+    values: ['revert']
   }
 }

test/cases/inheritance-keywords.css

@@ -0,0 +1,18 @@
+/*
+expect:
+css-initial-value: 1
+css-unset-value: 1
+css-revert-value: 1
+*/
+
+.initial-value {
+    color: initial;
+}
+
+.unset-value {
+    color: unset;
+}
+
+.revert-value {
+    color: revert;
+}