Added --disk-cache option

This option enables disk cache. If SIZE is 0, the disk cache is
disabled. This feature caches the downloaded data in memory, which
grows to at most SIZE bytes. The cache storage is created for aria2
instance and shared by all downloads. The one advantage of the disk
cache is reduce the disk seek time because the data is written in
larger unit and it is reordered by the offset of the file. If the
underlying file is heavily fragmented it is not the case.

test/SinkStreamFilterTest.cc

@@ -54,9 +54,7 @@ public:
   void setUp()
   {
     writer_.reset(new ByteArrayDiskWriter());
-    sinkFilter_.reset(new SinkStreamFilter());
-    filter_.reset(new SinkStreamFilter(sinkFilter_));
-    sinkFilter_->init();
+    filter_.reset(new SinkStreamFilter());
     filter_->init();
     segment_.reset(new MockSegment2(16));
   }