Added layer comments via 'metadata.maputnik:comment' (issue #28)

src/components/layers/CommentBlock.jsx

@@ -0,0 +1,24 @@
+import React from 'react'
+
+import InputBlock from '../inputs/InputBlock'
+import StringInput from '../inputs/StringInput'
+
+class MetadataBlock extends React.Component {
+  static propTypes = {
+    value: React.PropTypes.string,
+    onChange: React.PropTypes.func.isRequired,
+  }
+
+  render() {
+    return <InputBlock label={"Comments"}>
+      <StringInput
+        multi={true}
+        value={this.props.value}
+        onChange={this.props.onChange}
+        default="Comment..."
+      />
+    </InputBlock>
+  }
+}
+
+export default MetadataBlock