Add user and automatic configuration macro to disable exceptions

[mborland]

Closes: #971

[cppalliance-bot]

An automated preview of the documentation is available at https://972.decimal.prtest3.cppalliance.org/libs/decimal/doc/html/index.html

[cppalliance-bot]

An automated preview of the documentation is available at https://972.decimal.prtest3.cppalliance.org/libs/decimal/doc/html/index.html

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.8%. Comparing base (9501d6f) to head (c24610b).
⚠️ Report is 9 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #972     +/-   ##
=========================================
+ Coverage     98.8%   98.8%   +0.1%     
=========================================
  Files          249     249             
  Lines        16220   16220             
  Branches      1818    1818             
=========================================
+ Hits         16013   16014      +1     
+ Misses         207     206      -1     

Files with missing lines	Coverage Δ
include/boost/decimal/format.hpp	100.0% <ø> (ø)
test/test_big_uints.cpp	100.0% <ø> (ø)
test/test_decimal32_fast_stream.cpp	100.0% <ø> (ø)
test/test_decimal32_stream.cpp	100.0% <ø> (ø)
test/test_snprintf.cpp	100.0% <ø> (ø)
test/test_strtod.cpp	100.0% <ø> (ø)

... and 1 file with indirect coverage changes

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