Multi-line strings (#414)

* update based on discussion in #414

* Update versions/development/SPEC.md

Co-authored-by: Michael Franklin <michael@illusional.net>

* Add note that string interpolation works the same in regular and multi-line strings

* make example more clear

* update based on discussion in #414

* Update versions/development/SPEC.md

Co-authored-by: Michael Franklin <michael@illusional.net>

* make example more clear

* merge

* update based on discussion in #414

* Update versions/development/SPEC.md

Co-authored-by: Michael Franklin <michael@illusional.net>

* make example more clear

* fix

* fix

* fix some examples, clarify order of multi-line string and command template evaluation

* change multi-line string delimiter and whitespace rules

* remove old command section syntax; unify empty and blank line handling in multi-line strings

* typo

* make aside parenthetical

* remove outdated example

* reformat

* reformat

* reformat

* remove line continuations first, add example with line ending in escaped backslash

* add missing backticks

Co-authored-by: Michael Franklin <michael@illusional.net>