Add Params(map[string][]string) and Headers(map[string][]string) methods

[earthboundkid]

It would be nice to have a way to bulk set query params and the same for headers.

[earthboundkid]

From #76:

Sometimes you need to add parameters conditionally, and it's annoying to break the flow with an if statement, so having a Params method might be nice.

Questions:

• Should it be Params(map[string]string), Params(map[string][]string), Params([]string) (panic if len(strings) isn't even), or Params(...string)?
• Does it make sense to have Headers? Seems less useful.

[earthboundkid]

If it's map[string][]string, then you can potentially pass in url.Values or http.Header objects.

Could go crazy and do Params(any) and then figure it out from context. 😆 🤔

[earthboundkid]

Instead of Params(any), which is crazy, there could be requests.Map(any) which takes whatever and inflates it into a map[string][]string.

[tmm1]

👍 ran into this myself. An easy way to pass through url.Values would be appreciated.

[earthboundkid]

See #82.