The symbol argument is a three digits character which represents one currency.
The symbols can be obtained with list_currencies.
The time series date range is defined by start_date and end_date.
Value
The time series with the bid and ask currency rates regarding the given symbol quoted in BRL.
The default returning is a tibble-fashioned data.frame with
the three columns: date, ask and bid.
The as argument also accepts data.frame to return old fashioned data frames,
xts to return a xts object with two variables (bid and ask) and text which returns
the text content download from BCB site.