Accept Axios options

[dguo]

It would be helpful if the client could take an optional parameter for Axios options to pass to the actual Axios calls.

Recently, we have been getting a lot of ETIMEDOUT errors. Based on this StackOverflow answer, we added this:

axios.defaults.timeout = 30000;
axios.defaults.httpsAgent = new https.Agent({ keepAlive: true });

However, this applies to other places where we use Axios, since we're setting them globally. We can't selectively apply it only to the Synapse calls, and it's causing us issues. It would be great if we could pass in the options to the client constructor instead.

[jenstroeger]

I added options to the request itself. See #78.