Blacklight Plugin

This is a follow-on to #CODEBASE-220, which adds a new facet configuration parameter for controlling the facet partial rendered on a per-facet basis. This might be more contentious (maybe local implementations should override the name helper method and call it good..), but I thought I should offer it anyway.

The patch would add a configuration parameter to config/initializers/blacklight_config.rb like:
config[:facet] = {
  :field_names => [ ...],
  :labels => { ... },
  :limits => { ... },
  :templates => {
    "lc_callnum_display" => "catalog/_facet_partials/callnumbers"
  }
}

The patch is in http://github.com/cbeer/blacklight/commit/c120d82a4b3e29d2d549d612550ef5c0894e9d4e and attached.