stbt.conf: Move image-matching parameters to [match] section

(From the [global] section.)

This doesn't break backwards compatibility because these parameters
(match_method, match_threshold, confirm_method, confirm_threshold,
erode_passes) were introduced (in dd44303b) more recently than the last
stb-tester release (0.12).

The need for a separate section is that in future we may want to put the
parameters for *motion* detection into stbt.conf, and those (and other
new) parameters may conflict with the image-matching parameters.