Add .Params(map[string]string)

[earthboundkid]

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]

LOL, this is the same as #23.