Implement WithPgxConfig function

[acim]

In order to use plenty of available config for pgx connection (Tracer, DialFunc, etc.), it would be really nice to have this function. At the moment, there is no way to pass pgx connection to migrate, but passing connection configuration is good enough and also clean solution not affecting the rest of the migrate code.

[acim]

CI is failing, but it seems it is fixed in this PR https://github.com/golang-migrate/migrate/pull/810 so I didn't want to fix it again. If you merge that, I can sync with the upstream and check again if CI passes.

[xrn]

@acim can you rebase seems that #810 is merged

[coveralls]

Coverage decreased (-0.03%) to 57.885% when pulling cd4ae9760605f1b79782b245ff94b683dd46a382 on acim:with-pgx-config into 37a9174b9db87d8c7f65c72b8890e778e8d7d87d on golang-migrate:master.

[acim]

No, but you can define custom dialer or whatever option to the underneath pgx config. However, I solved my problem in a different way so you can close this if you don't like it, no problem.