highlight_html: improve performance
highlight_html: improve performance

It caches created keywords table while executing the expression.

[groonga-dev,03427]

Reported by Hiroyuki Sato. Thanks!!!

kenhys/groonga,komainu8/groonga,komainu8/groonga,kenhys/groonga,kenhys/groonga,groonga/groonga,cosmo0920/groonga,hiroyuki-sato/groonga,cosmo0920/groonga,hiroyuki-sato/groonga,hiroyuki-sato/groonga,groonga/groonga,cosmo0920/groonga,cosmo0920/groonga,naoa/groonga,naoa/groonga,naoa/groonga,groonga/groonga,komainu8/groonga,komainu8/groonga,groonga/groonga,groonga/groonga,naoa/groonga,hiroyuki-sato/groonga,cosmo0920/groonga,naoa/groonga,naoa/groonga,cosmo0920/groonga,naoa/groonga,cosmo0920/groonga,groonga/groonga,kenhys/groonga,hiroyuki-sato/groonga,hiroyuki-sato/groonga,groonga/groonga,kenhys/groonga,cosmo0920/groonga,komainu8/groonga,hiroyuki-sato/groonga,komainu8/groonga,hiroyuki-sato/groonga,kenhys/groonga,kenhys/groonga,komainu8/groonga,komainu8/groonga,naoa/groonga,kenhys/groonga,groonga/groonga