do variable substitutions in server urls

[michaelgwelch]

OpenApi allows for servers to define one or more server objects, each of which has a url that supports server variables. A server variable looks like a symbol in braces. For example {hostname}. A server object can also define variables which can provide values for any variables.

This commit honors any variables that have been defined which produces better looking code snippets. Otherwise server variables are escaped and produce snippets that look like:

var client = new RestClient("https:///%7Bhostname%7D/api/v5/login");