Implement style computing (#1399)

Ref https://github.com/svg/svgo/issues/777

Currently a lot of optimisations are attributes specific and may be
broken because of inline or shared styles.

In this diff I'm trying to solve the problem with getComputedStyle
analog.

`computeStyle` collects attributes, shared css rules, inline styles
and inherited styles and checks whether they can be statically optimised
or left as deoptimisation.

test/plugins/removeHiddenElems.01.svg

@@ -1,11 +1,20 @@
 <svg xmlns="http://www.w3.org/2000/svg">
+    <style>
+      .a { display: block; }
+    </style>
     <g>
-        <path display="none" d="..."/>
+        <rect display="none" x="0" y="0" width="20" height="20" />
+        <rect display="none" class="a" x="0" y="0" width="20" height="20" />
     </g>
 </svg>
 
 @@@
 
 <svg xmlns="http://www.w3.org/2000/svg">
-    <g/>
+    <style>
+        .a { display: block; }
+    </style>
+    <g>
+        <rect display="none" class="a" x="0" y="0" width="20" height="20"/>
+    </g>
 </svg>

test/plugins/removeHiddenElems.02.svg

@@ -1,11 +1,20 @@
 <svg xmlns="http://www.w3.org/2000/svg">
+    <style>
+      .a { opacity: 0.5; }
+    </style>
     <g>
-        <path opacity="0" d="..."/>
+        <rect opacity="0" x="0" y="0" width="20" height="20" />
+        <rect opacity="0" class="a" x="0" y="0" width="20" height="20" />
     </g>
 </svg>
 
 @@@
 
 <svg xmlns="http://www.w3.org/2000/svg">
-    <g/>
+    <style>
+        .a { opacity: 0.5; }
+    </style>
+    <g>
+        <rect opacity="0" class="a" x="0" y="0" width="20" height="20"/>
+    </g>
 </svg>

test/plugins/removeHiddenElems.12.svg

@@ -1,4 +1,12 @@
+Keep invisible elements which have visibile ones inside
+and resolve styles
+
+===
+
 <svg width="480" height="360" xmlns="http://www.w3.org/2000/svg">
+  <style>
+    .a { visibility: visible; }
+  </style>
   <rect x="96" y="96" width="96" height="96" fill="lime" />
   <g visibility="hidden">
     <rect x="96" y="96" width="96" height="96" fill="red" />
@@ -7,14 +15,19 @@
   <g visibility="hidden">
     <rect x="196" y="196" width="96" height="96" fill="lime" visibility="visible" />
   </g>
+  <rect x="96" y="96" width="96" height="96" visibility="hidden" class="a" />
 </svg>
 
 @@@
 
 <svg width="480" height="360" xmlns="http://www.w3.org/2000/svg">
+    <style>
+        .a { visibility: visible; }
+    </style>
     <rect x="96" y="96" width="96" height="96" fill="lime"/>
     <rect x="196.5" y="196.5" width="95" height="95" fill="red"/>
     <g visibility="hidden">
         <rect x="196" y="196" width="96" height="96" fill="lime" visibility="visible"/>
     </g>
+    <rect x="96" y="96" width="96" height="96" visibility="hidden" class="a"/>
 </svg>