adapt_framework: grunt add image compression command

[oliverfoster]

Subject of the issue

Sometimes it's nice to be able to web compress without having to open another tool.

[moloko]

Should also help with adaptlearning/adapt_authoring#2435

[moloko]

Could we have an option to run this on the src folder's graphics as well? That way you don't have to remember to keep running it every time you do grunt dev/build!

[moloko]

note to self: remember to document this command in GRUNT.md

[oliverfoster]

As agreed on the call:

grunt compress should work on course folder
grunt compress --outputdir= should work on specified folder