Drastically reduce body template weight.

Previously, InsertBody() would insert sth like this:

    {--------------------------}
    {syntax:rst}

    {--------------------------}

Now it's just:

    {syntax:rst}

    {----------}

I can't seem to get away without the last line -- my eyes seem to need
it (nothing syntactic).
LICENCE: Fix typos.
Update design documentation (with respect to testing).
Introduce tmux-based testing.

This commit includeds the complete infrastructure groundwork,

  + the tmux magic weaving run_tests,
  + a minimal vimrc.tests,

and one non-trivial test of the interlinking function.  I'm happy.
Fix one long-standing bug in JumpToTargetID().

The bug scenario is included in 1_interlinking.wt (third test).
Also: echo() -> echom() in most cases.