Add options to remove descriptive elements (#102)

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

unittests/full-metadata.svg

@@ -1,22 +0,0 @@
-<svg xmlns="http://www.w3.org/2000/svg">
-    <metadata>
-      <rdf:RDF
-           xmlns:rdf = "http://www.w3.org/1999/02/22-rdf-syntax-ns#"
-           xmlns:rdfs = "http://www.w3.org/2000/01/rdf-schema#"
-           xmlns:dc = "http://purl.org/dc/elements/1.1/" >
-        <rdf:Description about="http://example.org/myfoo"
-             dc:title="MyFoo"
-             dc:description="Unit test for Scour's --remove-metadata option"
-             dc:publisher="No One"
-             dc:date="2010-06-09"
-             dc:format="image/svg+xml"
-             dc:language="en" >
-          <dc:creator>
-            <rdf:Bag>
-              <rdf:li>No One</rdf:li>
-            </rdf:Bag>
-          </dc:creator>
-        </rdf:Description>
-      </rdf:RDF>
-    </metadata>
-</svg>