Extend GetTeamInfo API to accept team argument

[ed-wp]

https://api.slack.com/methods/team.info

team | string | to get info on, if omitted, will return information about the current  team. Will only return team that the authenticated token is allowed to see through external shared channels

The library omits this by default, but does not provide a way to specify another team.

Current:

func (api *Client) GetTeamInfo() (*TeamInfo, error) 

Proposed:

func (api *Client) GetTeamInfo(team string) (*TeamInfo, error) 

Expected behaviors:

If `team` == "" omit `team` from url.values
If `team` != "" add `team` to url.values