Add normalizer accessor to grn_snip

New functions:

  * grn_snip_set_normalizer()
  * grn_snip_get_normalizer()
Support adding conditions to existing grn_snip from grn_expr

New function:

  * grn_expr_snip_add_conditions()
snippet_html: use new grn_snip related API
Merge branch 'master' of github.com:groonga/groonga
test commit try grn_set_normalizer