MEDIUM: samples: add a regsub converter to perform regex-based transformations

MEDIUM: samples: add a regsub converter to perform regex-based transformations

We can now replace matching regex parts with a string, a la sed. Note
that there are at least 3 different behaviours for existing sed
implementations when matching 0-length strings. Here is the result
of the following operation on each implementationt tested :

  echo 'xzxyz' | sed -e 's/x*y*/A/g'

  GNU sed 4.2.1       => AzAzA
  Perl's sed 5.16.1   => AAzAAzA
  Busybox v1.11.2 sed => AzAz

The psed behaviour was adopted because it causes the least exceptions
in the code and seems logical from a certain perspective :

  - "x"  matches x*y*  => add "A" and skip "x"
  - "z"  matches x*y*  => add "A" and keep "z", not part of the match
  - "xy" matches x*y*  => add "A" and skip "xy"
  - "z"  matches x*y*  => add "A" and keep "z", not part of the match
  - ""   matches x*y*  => add "A" and stop here

Anyway, given the incompatibilities between implementations, it's unlikely
that some processing will rely on this behaviour.

There currently is one big limitation : the configuration parser makes it
impossible to pass commas or closing parenthesis (or even closing brackets
in log formats). But that's still quite usable to replace certain characters
or character sequences. It will become more complete once the config parser
is reworked.