Numtxt

Added si function (1234567890 -> '1.235 G'), version string, unmentioned functions (gen10_dict, gen1k_dict, table) and minor cleanup

added precedence (primary, secondary, tertiary, ...), (t)uple (single, double, triple, ...) and prefix functions. numtxt.log can now be overridden by third-party modules (e.g. gmpy2, mpmath) for compatibility:

>>> numtxt.log = gmpy2.log
>>> numtxt.approx(gmpy2.exp(1234567))
'432.155 octoseptuagintacentillivigintiseptingentillion'

approx now doesn't crash when given 0