adds support for OFFSET keyword

[bbeaudreault]

We already supported LIMIT <offset>, <limit> but not the LIMIT <limit> OFFSET <offset> construct, per http://dev.mysql.com/doc/refman/5.7/en/select.html

This adds support and a test for this case

@sougou

[bbeaudreault]

I should note that when we rewrite the query, we'll put it back to the LIMIT <offset>, <limit> syntax.

[sougou]

Can you resolve the conflict? Just need to regen sql.go.

[bbeaudreault]

Will do

[bbeaudreault]

Done, should be good once travis passes