Allow caller to set compression level and get last write offset.

This last is useful for building an index on the starting positions of
records as they are written. Due to buffering and blocking behavior
these are not necessarily the same as the positions reported by
calling #tell before each #write call.

lib/bio-bgzf/writer.rb

@@ -1,13 +1,21 @@
+require 'zlib'
+
 module Bio::BGZF
 
   class Writer
     include Bio::BGZF
 
-    attr_reader :f, :buf
+    attr_reader :f, :buf, :level
+
+    # Return the virtual offset of the last {#write} call. This is a
+    # hook for e.g. building an index on the fly.
+    attr_reader :last_write_pos
 
-    def initialize(f)
+    def initialize(f, level=2)
       @f = f
+      @level = level
       @buf = ''
+      @buf_write_pos = 0
       if block_given?
         begin
           yield self
@@ -24,22 +32,30 @@ def tell
     def write_buf
       if buf.size > 0
         raise "Buffer too large: #{buf.bytesize}" if buf.bytesize > MAX_BYTES
-        block = pack(buf)
+        block = pack(buf, level)
         f.write(block)
         @buf = ''
+        @buf_write_pos = tell
       end
     end
 
+    # @api private
+    def _cur_write_pos
+      @buf_write_pos + buf.bytesize
+    end
+
     def write(s)
       if s.bytesize > MAX_BYTES
         write_buf
+        @last_write_pos = _cur_write_pos
         _each_slice(s) do |slice|
           write(slice)
         end
       else
         if (s.bytesize + buf.bytesize) > MAX_BYTES
           write_buf
         end
+        @last_write_pos = _cur_write_pos
         buf << s
       end
     end