precondition: Minor wording tweaks
precondition: stbt.precondition has been renamed to as_precondition
Makefile: Recipes don't hide failures from `make`
Makefile: Fix stbt.html recipe for new pandoc

Ever since I upgraded pandoc to 1.12.2.1 (or possibly an earlier
version), the `stbt.html` we generate from `stb-tester/README.rst`
doesn't contain the information from the reStructuredText "field list"
at the beginning of the document:

    :Copyright: Copyright (C) 2012-2013 YouView TV Ltd. and others
    :License: LGPL v2.1 or (at your option) any later version (see LICENSE file in
              the source distribution for details)
    :Version: @VERSION@

The diff was:

    -  <title>stb-tester :: stbt: A video-capture record/playback system for automated testi
    +  <title>stb-tester :: stbt</title>
    @@
    -<dl>
    -<dt>Copyright</dt>
    -<dd><p>Copyright © 2012-2013 YouView TV Ltd. and others</p>
    -</dd>
    -<dt>License</dt>
    -<dd><p>LGPL v2.1 or (at your option) any later version (see LICENSE file in the source distribution for details)</p>
    -</dd>
    -<dt>Version</dt>
    -<dd><p>0.17</p>
    -</dd>
    -</dl>

Note 2 things have gone: The subtitle, and the definition list.

Upon further investigation, nothing before the first proper heading is
shown (the subtitle doesn't count as a heading). The solution is to
change the subtitle formatting so that it looks like a heading. That is,
from this:

    --------
    subtitle
    --------

to this:

    subtitle
    ========

It has to be "=====" and not "-----"; otherwise all other "====="
headings in the document turn into `<h3>` instead of `<h2>`.

I didn't change the source document (`README.rst`) because stb-tester's
Makefile generates the `stbt.1` man page from `README.rst` using
a *different* tool (`rst2man` from docutils) and I don't want to screw
up *that* tool's output.
index.md: Add OCR to feature list

OCR was added in stb-tester 0.18.