Figwheel support

[artemyarulin]

REPL is nice, but with Buck we broke usual Figwheel workflow - saving file doesn't do anything now as we are serving REPL from a different folder with source copies.

We cannot simply rerun build as it will clean up output folder first so all JS files gonna disappear.

Previously we had some fragile script which looks something like that:

project_dir=`buck build $2 && buck targets $2 --show-output | awk '{print $2}'`
tmp=`mktemp -d`
rsync -r $project_dir/ $tmp/
watchman-make -p 'lib/**/*' 'RULES/**/*' --make "buck build $2 && rsync -r $PWD/$project_dir/ $tmp/" -t '' & (cd $tmp && lein repl) && kill $!