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

```
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?