select: Support dynamic columns

[kou]

The current Groonga can't use computed value for output_columns, drilldown and sortby.

If Groonga supports dynamic columns, we can use computed value for them. Because users just put computed values to dynamic columns. Dynamic columns can be implemented by columns for temporary table (result table is a temporary table).

There is another feature provided by dynamic columns. Naming computed value in output_columns. For example, --output_columns 'snippet_html(column1), snippet_html(column2)' uses snippet_html for both computed values for now. If Groonga supports dynamic columns, we can use another label for each computed value.

API idea:

select \
  --column[name].stage filtered \
  --column[name].type ShortText \
  --column[name].flags COLUMN_SCALAR \
  --column[name].value 'column1 + column2'

Available stage values:

• initial: Created a column at the first. It can be used in query, filter, drilldown, sortby and output_columns.
• filtered: Created a column after query and filter are evaluated. It can be used in drilldown, sortby and output_columns.
• output: Created a column after sortby. The value of column should only evaluated only for limit records.

[kou]

filtered is implemented at master.

[s-yata]

Dynamic columns seem to be already supported.
It uses --columns[name].xxx instead of --column[name].xxx.