Allow customizable switch boundaries for scientific notation.

[maisonobe]

This pull request allows users to customize the limit between decimal and scientific notation.
The current version of Ryū is compliant with the Java spec and switches to scientific notation
for numbers smaller than 10⁻³ or larger than 10⁺⁷. This is cumbersome in some cases where
scientific notation is forbidden. One example is formatting the seconds in a date obeying
ISO-8601 standard (or equivalently CCSDS 301.0-B-4 time code formats). In these cases,
time in the first millisecond of each minute would be produced in scientific notation. This would
imply using a printf-like format (but Ryū-printf is not available in the Java language) or rely on
the standard formats with a predefined accuracy, instead of the full-preserving property of Ryū.

The proposed changed simply uses user-customizable values for the limit exponents (-3 and +7
by default), while still preserving the existing API for regular uses.

Of course, this only shifts the problem to a smaller exponent, as in the example above even setting
lowExp to -12 would mean the problem arise again in the first picosecond of each minutes. This
however can be managed at caller level, by truncating too small values according to the domain.

In the date example above, we know that in the GNSS field absolute dates are accurate at about
nanosecond level, and we can decide to truncate at picosecond level so no accuracy is lost in
write/parse loops. However, inaccuracies at millisecond level are far too large (as signal travels
at the speed of light, 1ms corresponds to 300km…).