Reimplement incremental builds on watch command

Problem/Motivation

A few features were lost when the build tools were refactored a while back. This issue is to track bringing those back.

1. No source files are compiled when starting the watch process. This becomes more of an issue in codebases where the built assets are ignored from the source repository. In these cases, a dev may pull the latest changes but won't actually see the updated build assets unless they run the build command.
2. While running watch, if a CSS file change is detected, all CSS files are compiled. Likewise with JS. This becomes a performance issue in larger codebases resulting in multiple second compile times on each save.
3. When saving a dependency, such as a Sass partial file, the depending files are not recompiled. This requires an extra file save to trigger a file to be compiled.

Updating existing themes

If you used the generator to generate a custom Prototype theme from the 5.x version, you can manually update your tools by doing the following.

1. Add new dependencies npm install -D vinyl-named gulp-dependents
2. Remove unneeded dependencies npm remove gulp-plumber
3. Replace your gulpfile with this one