add an option to merge default configs

[monkeyWzr]

Fix #314

[gjtorikian]

Heya! This looks good, but I'm about to head on vacation and the only change I'd request is for merge_default_options to be removed. You're right in that it's more natural/correct to expect that the rest of one's defaults remain unchanged; so the default here should be to always merge in the defaults (and respect the user's config choice otherwise).

[monkeyWzr]

Enjoy your vacation!

the only change I'd request is for merge_default_options to be removed

Sure! I'm happy to make the changes but there are two things:

• tons of tests need to be fixed
• it might break user's app if someone is relying on the current behavior

For the first one I think I can find a way out. For the second I really don't know what I can do so I'm gonna leave it to you 😄 (maybe some breaking change notes in the changelog?)