Add parser for Optional types to avoid the null madness

[lucidd]

This adds a parser for Optional<T> for all types T we can already parse.
The main motivation behind this is to banish null as much as possible in favor of the much saver abstraction Optional<T> which denotes a possibly absent value. This addresses concerns raised in #77

[lucidd]

I just rebased this to the latest master. I would say we merge this now as is even if we are not able to completely enforce that non optional values are never null its still valuable to mark Optional values as such and have them parsed accordingly.

[renner]

We can merge this if you need it, choose yourself if you want to merge it from here or include the commits with #109.

[renner]

Let's put only one empty line between methods (here and above this method).

[lucidd]

I would prefer merging this PR first i will rebase #109 accordingly.