Add missing words, punctuation in input.md
Merge pull request #52 from robinst/patch-2

Add missing words, punctuation in the option chapter.
update.rs: ~"" is gone in latest rust
option: improve description of Option
loops: show how to use break from nested loops
trait: replace syntax extension with attribute
varibles: improve wording
tuples: don't use unexplained concepts. Closes #45
generics; show how to specialize generic functions
trait: Reword first paragraph a bit