Added DialWithOptionsAndLocal method

[ProtossGenius]

Some requirements may need to specify a local address, such as intranet penetration

[ProtossGenius]

Some requirements may need to specify a local address, such as intranet penetration

[codecov-io]

Codecov Report

Merging #153 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #153      +/-   ##
========================================
+ Coverage   75.98%    76%   +0.02%     
========================================
  Files          10     10              
  Lines        2082   2084       +2     
========================================
+ Hits         1582   1584       +2     
  Misses        418    418              
  Partials       82     82

Impacted Files	Coverage Δ
sess.go	84.9% <100%> (+0.05%)	⬆️

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[ProtossGenius]

能够指定本地地址在某些时候或许会很有用，好像UDP内网穿透的时候就是从使用指定的端口的连接服务器开始的。

[xtaci]

为什么不用NewConn传入拨好的udpconn呢

[ProtossGenius]

实际写起来的时候行数会有一点差别
用NewConn应该至少要写6～7行，新加一个方法的话一行就能搞定，最主要的是这样这个就能很容易的打洞了。

//外面拨的话会额外写 这么多的代码，基本上要写
udpaddr, err := net.ResolveUDPAddr("udp", raddr)
	if err != nil {
		//错误处理
	}
	network := "udp4"
	if udpaddr.IP.To4() == nil {
		network = "udp"
	}

	conn, err := net.ListenUDP(network, &net.UDPAddr{})
	if err != nil {
              //错误处理
	}
       conn, err := NewConn(raddr, block, dataShards, parityShards, conn)

不过就算不加的话在外面封装一个方法就行了，算是可有可无的吧。