format.h doesn't handle `char*` correctly

[jrabinow]

char *foobar = "lorem ipsum";
format("dolor sit {1} amet", foobar);

Expected result:
"dolor sit lorem ipsum amet"

Actual result:
"dolor sit dolor sit amet amet"

When passing in a char*-type variable to format's varargs list, need to cast the variable to std::string or convert it to a std::string. Not doing so results in fmt variable (first arg) being read a second time as one of the vararg variables.

Not a major issue but this small wart can take people by surprise - if this is wontfix, may I recommend adding a comment in the source?