std::basic_string allocator support

Modified the constructors of BasicStringRef and BasicCStringRef to accept std::basic_strings with any allocator. Also modified and BasicWriter.str() to accept custom allocator parameter. This allows the following code to work:

using scalable_string = std::basic_string<char, std::char_traits<char>, scalable_allocator<char> >;
fmt::MemoryWriter writer;
writer << "Hello from " << scalable_string{"fmtlib"} << '\n';
const scalable_string result_str = writer.str<scalable_allocator<char> >();

Modified StringBuffer and BasicStringWriter to accept custom allocator template parameters for the internal string and the move_to parameter.