Fix tests/while-1.txt
Update ReadMe
Update ReadMe: introduction
Use dict(zip(...)) in ForBlock.run get_for_targets
Update PypageExec.run

Disallow ';' in single-line code tags for consistency and to avoid
problems such as quoted semicolons. (We can't use exec instead since we
need a return value through eval.)

In addition, slightly updated, how a {{ write(...) }} is handled.
Updates to ReadMe
Centralize escape delimiters
Update ReadMe.rst (minor)