Support special floating point values: nan, inf.

[Tim Finer]

The special values (taken from the TOML github page), are not parsed correctly:

infinity

sf1 = inf # positive infinity
sf2 = +inf # positive infinity
sf3 = -inf # negative infinity

not a number

sf4 = nan # actual sNaN/qNaN encoding is implementation specific
sf5 = +nan # same as nan
sf6 = -nan # valid, actual encoding is implementation specific