Add --tags flag support to serve command.
Add --tags flag support to serve command.

Currently, only build and install commands make use of the tags flag to
set custom build tags. Add this functionality to serve command, so it's
possible to use custom build tags with serve.

govlas/gopherjs,ebroder/gopherjs,Archs/gopherjs,flimzy/gopherjs,rasata/gopherjs,govlas/gopherjs,flimzy/gopherjs,jeremyschlatter/gopherjs,nmfzone/gopherjs,albrow/gopherjs,Yanjing123/gopherjs,roth1002/gopherjs,Yanjing123/gopherjs,Yanjing123/gopherjs,bpowers/gopherjs,flimzy/gopherjs,jeremyschlatter/gopherjs,ebroder/gopherjs,jargv/gopherjs,gopherjs/gopherjs,Archs/gopherjs,roth1002/gopherjs,nmfzone/gopherjs,roth1002/gopherjs,ebroder/gopherjs,albrow/gopherjs,bpowers/gopherjs,jargv/gopherjs,albrow/gopherjs,jargv/gopherjs,govlas/gopherjs,gopherjs/gopherjs,bpowers/gopherjs,gopherjs/gopherjs,rasata/gopherjs,gopherjs/gopherjs,nmfzone/gopherjs,gopherjs/gopherjs,jeremyschlatter/gopherjs,Archs/gopherjs,rasata/gopherjs