Add keyword search placeholder option for Finders

For some Finders "eg `document.noun` title` doesn't make sense. This
commit allows us to specify a keyword_search_placeholder in the schema
and have it used for the keyword facet. It default to using
document.noun if this isn't present.
Merge pull request #103 from alphagov/add-keyword-search-placeholder-option-for-finders

Add keyword search placeholder option for Finders