treasury 0.6.0

The daily interest rate functions now include an updated_at column with the feed’s last update time.
The daily interest rate functions now reject an invalid date, such as a month outside 01-12, with an informative error instead of silently returning no data.
tr_bill_rate() now includes maturity_date and cusip columns identifying the bill quoted for each maturity tranche.
tr_curve_rate(), tr_par_yield(), and tr_forward_rate() now parse dates correctly regardless of the session’s locale (previously failed under non-English locales).
tr_long_term_rate() now includes an extrapolation_factor column with the adjustment factor used to estimate 30-year rates between 2002 and 2006 (NA outside of that period).
tr_yield_curve() now correctly labels the 1.5-month maturity (previously shown as 1 _5month).

treasury 0.5.0

Add optional caching of API responses via options(treasury.cache = TRUE). Cached responses are stored for 1 day by default and can be customized with options(treasury.cache_max_age = seconds). Use tr_cache_dir() to find the cache location and tr_cache_clear() to clear it.

Fixed bug in checking for {readxl} installation.
Improved documentation.
Renamed functions for consistency:
- tr_bill_rates() to tr_bill_rate()
- tr_par_yields() to tr_par_yield()

Migration to data.table package. Internal data manipulation is now done using data.table and all functions return data.table objects.