Add support for skipping the self-export step

[colans]

Currently, the destination's configuration is exported and committed before merging configuration from the source.

There are situations where this behaviour is not desirable, when the destination's database configuration is irrelevant/throw-away (say on a testing site) or the developer has already done an export and committed exactly what he/she wants from it.

So there are times when we only want to merge branch code on the destination with exported database configuration from the source, without needing anything from the destination's database.

As such, I'm proposing a --no-self-export option which, when provided, will skip the destination export and commit.

I'm working on this currently, and am planning to have a PR for this soon.

[colans]

Perhaps this is no longer necessary now that there's a config-pull command. See drush-ops/drush#1711 for details.