add --dryrun to wp-cli

[Oliver Jones]

Make it so we can say

wp index-mysql enable --all --dryrun

(or disable or upgrade) and get an output of the DDL needed to change the indexes. Make that output so we can do

wp index-mysql enable --all --dryrun | wp db query

and run the DDL directly (meaning: make the --dryrun output valid sql with sql comments in headers).

[Oliver Jones]

Fixed for 1.4.5.