Multiple operations done in one API call

Previously, each operation was performed with a separate API call,
resulting, in the worst case scenario, in the user going over a free
Weather Underground API key's 10 calls per minute limit with only one
--all request.  By combining all of the Conditions structs and changing
the necessary functions to take slices as arguments, wu makes only one
API call per run.

BUG: Weather Underground doesn't permit a history search and a planner
search at the same time; this confluence of flags raises an error
message.