Add options to remove descriptive elements (#102)

* --remove-titles (removes <title>)
* --remove-descriptions (removes <desc>)
* --remove-descriptive-elements (removes <title>, <desc> and <metadata>)

unittests/descriptive-elements-with-text.svg

@@ -1,4 +1,6 @@
 <?xml version="1.0" encoding="UTF-8" standalone="no"?>
 <svg xmlns="http://www.w3.org/2000/svg">
+  <title>This is a title element with only text node children</title>
+  <desc>This is a desc element with only text node children</desc>
   <metadata>This is a metadata element with only text node children</metadata>
 </svg>

unittests/empty-descriptive-elements.svg

@@ -1,3 +1,5 @@
 <svg xmlns="http://www.w3.org/2000/svg">
+  <title></title>
+  <desc></desc>
   <metadata></metadata>
 </svg>

unittests/full-descriptive-elements.svg

@@ -1,4 +1,13 @@
 <svg xmlns="http://www.w3.org/2000/svg">
+    <title xmlns:mytitle="http://example.org/mytitle">
+      <mytitle:title>This is an example SVG file</mytitle:title>
+      <mytitle:desc>Unit test for Scour's --remove-titles option</mytitle:desc>
+    </title>
+    <desc xmlns:mydesc="http://example.org/mydesc">
+      <mydesc:title>This is an example SVG file</mydesc:title>
+      <mydesc:para>Unit test for Scour's
+      <mydesc:emph>--remove-descriptions</mydesc:emph> option</mydesc:para>
+    </desc>
     <metadata>
       <rdf:RDF
            xmlns:rdf = "http://www.w3.org/1999/02/22-rdf-syntax-ns#"