0.4.3

• Added argagg::fmt_string() to format a string using the fmt program when
  compiling on a unix platform. Degrades to an identity function when not
  on a unix platform.
• Added argagg::fmt_ostream as a convenience stream that will stream the
  formatted string to the referenced final std::ostream when the
  argagg::fmt_ostream destructs
• Fixed bug when compiling with clang where the non-template implicit bool
  operator was being selected when implicitly converting to an integer. By
  making the implicit bool operator a specialization of the templated implicit
  conversion operator the compiler should now select the correct overload.