Mi & save/load: add option to indent or not on config writer creation

The presence of tabulation and newline in mi is irrelevant and can
impact client performance. The parsing & processing of a lot of data
from mi with many /t and /n impact performance. This aim to fix the
problem at the source. Style is useless for machine.

Signed-off-by: Jonathan Rajotte Julien <jonathan.r.julien@gmail.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>