Add highlight_full() function

It tags output text. It can specify several options and multiple keywords.

    highlight_full(column, normalizer_name, html_escape_flag,
                   keyword1, open_tag1, end_tag1 ... [keywordN, open_tagN, end_tagN])

TODO: Document it.
Add grn_expr_get_keywords()

It gets keywords from expression.

TODO: Document it.
Add highlight_html() function

It tags output text after it html_escape and normalize text. It can use easily without option.

    highlight_html(column)

TODO: Document it.
test highlight_full: remove a needless error message
Merge pull request #185 from naoa/add-highlight_html-function

Add highlight_full() function and highlight_html() function

Patch by Naoya Murakami. Thanks!!!