Contour line's elevation value sometimes hard to read

[sletuffe]

Related to #2
but also valid for contour line's elevation even on the bright sides of mountains : font size is really tiny, and especially on smartphone hard to read.

With raster rendering there is no dynamic font size selecting, so that is a trade-off, but I'm incline to raise it just a bit :
Currently a 7 : https://github.com/sletuffe/OpenTopoMap/blob/master/mapnik/styles-otm/contours.xml#L25
Might take a stab at 8 or 9

[sletuffe]

Juste to give insight on the following of this issue, on my repo I've tried increasing the white halo around contour's labels, and increasing font size by +1 and things are much readable now !

The question is now : should I import those change on the OpenTopoMap as well ?