numeric(), integer() or character(),
vector of starting years of desired seasons in YYYY
format, e.g. 1995 or "1995" for season 1995-1996.
Accepts vectors such as c(1995:2000, 2010) to generate
multiple seasons.
Alternatively, also accepts character() with seasons in the
format "YYYYZZZZ", where ZZZZ = YYYY + 1, e.g. "19951996".
This is the format that ultimately gets sent to the NHL API.
Some API endpoints, notably seasons exposed via nhl_seasons()
also allow the value "current" to passed. This value will be
returned unchanged.