Documentation and renaming of "blankies" into somethign a bit easier to ...

...find.

* Rename SplitExpression* to `SbtParser` denoting that is parses .sbt files
* Adds a few todos
* Document APIs for internal usage.

Review by @ajozwik  cc> @eed3si9n 
Why we need lines? File is enough - we can read in `SplitExpressionsNoBlankies`. But I think it is for future changes.