Add flagfile option to CompilerJarFilter

The --flagfile option to Closure defines a file containing additional flags formatted like this:

    --define=TESTING
    --externs="../lib/externs/es3.js"
    --externs="my-custom-externs.js"

Currently Assetic's Closure wrapper implements a subset of the possible options and in some cases doesn't allow multiple options (e.g., setJsExterns only allows setting one file, whereas it can be specified multiple times on the command line).  Adding just the --flagfile option makes it possible to include all possible options, multiple options, and options added in the future without needing to first modify the CompilerJarFilter class.