Add compile_css and build_js options to compile_assets task

so that canvas-plugins-aux can run only js:generate, canvas:check_syntax, and doc:api tasks.

Both options should default to true.

refs https://trello.com/c/h8kAol4y

Change-Id: If8aedcbd4f0a2d8cdcb9be7cfcaf486235d6d239
Reviewed-on: https://gerrit.instructure.com/43595
Reviewed-by: Jon Jensen <jon@instructure.com>
Product-Review: Jon Jensen <jon@instructure.com>
QA-Review: Jon Jensen <jon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>