build.py --force needed when customizations change

[joeyparrish]

When customizing the build as detailed in the Plugins and Customizing the Build Tutorial, if you change the build parameters like so:

# Build without polyfills:
python build/build.py +@complete -@polyfill
# Build without polyfills or text parsers:
python build/build.py +@complete -@polyfill -@text

The second build will not happen without --force. Instead, you get this message:

Compiling the library...
No changes detected, not building.  Use --force to override.

Ideally, we could detect that the build parameters have changed and rebuild anyway.

[joeyparrish]

Design from @TheModMaker:

1. Next to each build, output a manifest of the input JS files that went into that build.
2. When asked to build, check for this manifest and compare it to the list of inputs files for this build.
3. If the list has changed, rebuild regardless of changes to the source files.