Use unsigned type for integer digit

Fixes #109. Although we always used numbers in the ASCII range, it was
in a `size_t`, leading to a warning about possible loss of data on some
compilers.

CHANGELOG.md

@@ -9,6 +9,10 @@ options. The project adheres to semantic versioning.
 
 * Allow integers to have leading zeroes.
 
+### Bug Fixes
+
+* Fix a warning about possible loss of data.
+
 ## 2.1.1
 
 ### Bug Fixes

include/cxxopts.hpp

@@ -547,7 +547,7 @@ namespace cxxopts
 
       for (auto iter = value_match.first; iter != value_match.second; ++iter)
       {
-        size_t digit = 0;
+        US digit = 0;
 
         if (*iter >= '0' && *iter <= '9')
         {