Unify handling of texts of meaning and purposes

[hanjoosten]

Currently, purposes can contain texts in different kinds of layout, like markdown, LaTeX, ReST, etc, We use pandoc to parse these texts. We also use pandoc to render the documents created with Ampersand. In ADL, we use {+ and -} as start and end of such texts.

The following changes would be nice:

• [x] Change -} into +}. This makes it more obvious that the begin and end of a pandoc string belong together. It also allows for nested block-comments. (this was broken because of the use of -}. This will require a change in the lexer.
• [ ] Enable the use of pandoc texts in MEANING statement. This will probably require cleaning up some code in Motivations.hs