Added passing in build flags to the go build command

May be used by the revel run, build, package, test or debug commands to send specific flags
to "go build" like -ldflags -extldflags=-fno-PIC which would allow you to resolve issue #631 in the booking app on some machines. Compatible with existing code since arguments may be 0 length and build will work

Signed-off-by: NotZippy <notzippy@gmail.com>