Summary: svformat explicitly takes a container for direct indexing,
but the format string used is per-arg, which corresponds to format/sformat.
Closes #228
Reviewed By: @JoelMarcey
Differential Revision:
D2184334
// And if you just want the string,
std::string result = svformat("The only {what} is {value}", m);
// => "The only answer is 42"
-std::string result = svformat("The only {0[what]} is {0[value]}", m);
+std::string result = sformat("The only {0[what]} is {0[value]}", m);
// => "The only answer is 42"
// {} works for vformat too