Add Support for Custom verify command

[theory]

If, for example, you wanted to run verify scripts through pg_prove instead of psql, you could set it up with something like:

sqitch config verify.client 'pg_prove -U %u -h %h -p %p -d %d %f'

The idea is to have formatting characters stand in for options, i.e.,

• --db-user: %u
• --db-host: %h
• --db-port: %p
• --db-name: %d
• $script: %f