Added struct fmt::FormattingOptions

This allows to build custom `std::Formatter`s at runtime.

Also added some related enums and two related methods on `std::Formatter`.

library/std/src/lib.rs

@@ -292,6 +292,7 @@
 #![feature(dropck_eyepatch)]
 #![feature(f128)]
 #![feature(f16)]
+#![feature(formatting_options)]
 #![feature(if_let_guard)]
 #![feature(intra_doc_pointers)]
 #![feature(lang_items)]