SVG recoloring and some Gruntfile tidying

The SVG originals are copied from core Moodle then minified with
svgo (saving about 50% size).

For whatever reason is seems to take two refreshes for the images
to update to their new color, even after running decache.

In the gruntfile I used the simpler src/dest file format for things
with only 1 input and 1 output file.

I also added the ability to specify MOODLE_DIR as a environment
variable.