feat(bower): use exposify to build bower bundle

This commit uses exposify to perform a quick replace on web dependency
require() statements before browserify can process them.

This way we access snapsvg and lodash via window.Snap and window._
respectively during a bowerized build to ensure we can properly
integrate into web applications as a standalone library.

This approach is compatible with building downstream bpmn-js bower
packages.

Related to #37
docs(diagram): update plug-in documentation

Related to #38