allow mixins for bootstrap context

@peakxu 

Fix the busted hack of monkey patching the bootstrap context. Instead a mixin can be defined in the config.yml,

    bootstrap_mixins: Pipeline::BootStrapContext

or multiple
 
   bootstrap_mixins: 
       - Pipeline::BootStrapContext
       - Support::AnyModuleReally

When the pipeline is deployed, the configured mixins are required and included in the `BootstrapContext` used to compile the templates erbs.