Added an option to create footers in the table component

[francesco-cattoglio]

As per issue #860 this pull request implements two new parameters for the table component: first_row_is_footer and freeze_footers

[lovasoa]

Could we simplify things by introducing a new row-level property _sqlpage_footer and asking that the footer comes in last ?

So that we could do

select 'table' as component;

select col1, col2 from my_table;

select true as _sqlpage_footer, sum(col1) as col1, sum(col2) as col2 from my_table;

[francesco-cattoglio]

Sure thing! I actually think that we could even work around the requirement of the footer being the last row, since any table can have multiple <tbody>s and should still work correctly.

[francesco-cattoglio]

I implemented the requested change trying to keep the total modifications from the original code as little as possible.
This currently always produces an empty <tbody></tbody> when footer is used but everything renders and works correctly AFAICT.
If you want I can hack around a little more to enforce/ensure that the only correct way to use this is to have the _sqlpage_footer as the last row, and no empty <tbody> sections are created

[lovasoa]

thank you @francesco-cattoglio !