feat(mergeConfig): add defaultFinalizeContext

[stevemao]

Makes it possible to modify the result based on the default function.

Not sure if this is a good idea?

[coveralls]

Coverage increased (+0.02%) to 96.154% when pulling b6f6f7c on defaultFinalizeContext into 35285bf on master.

[coveralls]

Coverage increased (+0.02%) to 96.154% when pulling b6f6f7c on defaultFinalizeContext into 35285bf on master.

[stevemao]

Default values should only be simple primitive values. Functions shouldn't have defaults. Should probably look into how API should be designed again.

[stevemao]

The API should probably look something like expressjs's middleware, promise or stream pipe where you can do something else based on the previous work.

[stevemao]

Closing in favour of conventional-changelog-archived-repos/conventional-changelog-writer#20