Allow disabling integer to floating-point narrowing conversions for...

Allow disabling integer to floating-point narrowing conversions for cppcoreguidelines-narrowing-conversions

This change adds an option to disable warnings from the
cppcoreguidelines-narrowing-conversions check on integer to floating-
point conversions which may be narrowing.

An example of a case where this might be useful:
```
std::vector<double> v = {1, 2, 3, 4};
double mean = std::accumulate(v.cbegin(), v.cend(), 0.0) / v.size();
```
The conversion from std::size_t to double is technically narrowing on
64-bit systems, but v almost certainly does not have enough elements
for this to be a problem.

This option would allow the cppcoreguidelines-narrowing-conversions
check to be enabled on codebases which might otherwise turn it off
because of cases like the above.