Accidentally deleted dist folder breaks the workflow

[simplesmiler]

Accidentally deleted dist folder breaks the workflow, because browserify and watchify do not and seemingly will not ensure that the output dir exists: browserify/watchify#269

One approach would be to add a dev dependency on mkdirp, and then mkdirp dist && ... in the watchify and build npm scripts. I'll make a PR for both browserify and browserify-simple templates if this is a sound approach.