Adds support for decoding and encoding data with missing value indicators.

As part of the a whole bunch of refactoring was also done.

* Added support for `none` string to indicate missing values for both
  Encode and Decode.
* Removed decode sources. Instead the only decode source is now a list
  of strings.
* Changed encode to encode to a list of strings.
* Changed decode to only decode a list of strings.
* Removed `Val`s.
* Removed `Cell` it wasn't used anywhere and depended on `Val`.
* Added documentation to Encode and Decode.
* Added tests for encode macro.

This closes #76.
I agree that a predicate would be more flexible.

I think having a type that encapsulates both `String => Option[String]` and its dual would increase complexity quite a bit for little gain.

I'm inclined to leave it as is and when we need to make the change it won't be too hard to make.