Added "savestepsize" configuration option to allow modification of luacov.stats file updating frequency.

This is necessary for short lived scripts to avoid losing coverage data.
See: https://groups.google.com/forum/#!msg/openresty/CGN5CcnVjKI/k9nae1PraYUJ
Merge pull request #20 from kipras/master

Added "savestepsize" configuration option to allow modification of luaco...