Docgen: Re-colourise jsduck warnings.

When jsduck is executed in a subshell (as we do, to catch output)
it disables the color output. It should have a --color flag, but
it doesn't have that, so manually colourise for now.

Change-Id: I1f8da523b6b4fc1fc6965f751395f8af497b13b5
Merge "Docgen: Re-colourise jsduck warnings."