Added variadic versions for Format that act as a proxy for providing an initialiser list to a BasicFormatter.

Updated the Variadic test to use these new functions and verified that the tests passed.

See https://github.com/vitaut/format/issues/25

Fixed some places in BasicFormatter where non-templated string types were being used

See https://github.com/vitaut/format/issues/24

Thanks https://github.com/jose55 for the idea.

Expose in-place decimal integer formatting via FormatDec. Improve performance of CountDigits based on the results from https://github.com/localvoid/cxx-benchmark-count-digits.