Add support for specifying version of documents

[huiyifyj]

Describe the PR
Add --version flag for init subcommand, to specify the version of the API in the generated files.

This can reset the version of API docs with external command lines, such as the following:

$ swag init --version $(git describe --tags `git rev-list --tags --max-count=1`)

[ubogdan]

I like the idea so far. This will actualy override the version described in annotation ?
Maybe we should log a warning message in console if the version was set in annotation.

	if config.Version != "" {
                if swagger.Info.Version  != "" {
		    log("Warn: documentation version got overrided by flag --version")
	        }
		swagger.Info.Version = config.Version
	}

Please update Readme.md.

[huiyifyj]

@ubogdan 👌, done!