Add answers_at_end option and review behavior for solutions_at_end

[aless80]

The --solutions_at_end option is meant to place solutions to exercises at the end of the document. The --without_solutions and --without_answers remove solutions or answers from the main part of the document:

--solutions_at_end: Place solutions to exercises at the end of the document.
--without_solutions: Leave out solution environments from exercises.
--without_answers: Leave out answer environments from exercises.

With reference to the points below I request to review some of the logic and formatting. I also request to implement an analogous --answers_at_end option.

• --solutions_at_end works only when --without_solutions is used. This is not mentioned in the docs. UI propose making the --without_* and --*at_end options work independently for the main text and last chapter, respectively.

• --solutions_at_end together with --without_solutions shows answers in the end of the document. Also, the formatting for answers is different (eg it shows a bold 'Answer.').

• Subexercises are introduced by a bold letter such as a) followed by a newline. Without newlines the resulting document is more visually appealing.

• --solutions_at_end, --without_solutions, --without_answers shows only solutions in the end of the document. This could make sense, but I propose introducing a --answers_at_end option to provide disambiguation and more flexibility.