support for more precise geocode (closes #25)

[PMassicotte]

> data("countries")
> head(countries[countries$code == "CA", ])
    code country subcode                   subname
652   CA  Canada   CA-AB                   Alberta
653   CA  Canada   CA-BC          British Columbia
654   CA  Canada   CA-MB                  Manitoba
655   CA  Canada   CA-NB             New Brunswick
656   CA  Canada   CA-NL Newfoundland and Labrador
657   CA  Canada   CA-NS               Nova Scotia

> head(gtrends("nhl", geo = "CA-AB")$trend)
       start        end nhl
1 2004-01-04 2004-01-10  17
2 2004-01-11 2004-01-17  18
3 2004-01-18 2004-01-24  15
4 2004-01-25 2004-01-31  12
5 2004-02-01 2004-02-07  16
6 2004-02-08 2004-02-14  18

[eddelbuettel]

Awesome!