Add mysql connection character set option.

[name212]

Hi!

Openresty mysql driver https://github.com/openresty/lua-resty-mysql provides the ability to specify a character set for a connection using the 'charset' option.

Is it possible to add this parameter to the lapis configuration?

[edubart]

You can already specify charset using driver options in resty_mysql field as seen in this commit (master only atm) https://github.com/leafo/lapis/commit/ff23e41dd4baa818a6334bf92fc037c75ff73739