Add MODELTRANS_DEFAULT_LANGUAGE setting

[jacobwegner]

See use case in #116.

I worked with the existing test suite using the following patch and verified that everything still passed as expected:

diff --git a/tests/app/settings.py b/tests/app/settings.py
index a0adb93..5229cd3 100644
--- a/tests/app/settings.py
+++ b/tests/app/settings.py
@@ -90,8 +90,8 @@ DEFAULT_AUTO_FIELD = "django.db.models.AutoField"
 # Internationalization
 # https://docs.djangoproject.com/en/1.11/topics/i18n/
 
-LANGUAGE_CODE = "en"
-
+LANGUAGE_CODE = "en-US"
+MODELTRANS_DEFAULT_LANGUAGE = "en"
 MODELTRANS_AVAILABLE_LANGUAGES = ("nl", "de", "fr")
 
 TIME_ZONE = "UTC"

I also added DefaultLanguageConfTest to verify that settings override got picked up.

Please let me know if there are any additional tests / documentation that would be required.

[jieter]

The use case seems to be fair, and the implementation doesn't add much complexity 👍

I think we should add this setting to the documentation. Can you extend the PR with documentation for this setting?

[jacobwegner]

@jieter Thanks for the feedback. I did a pass to update the documentation and link to #116 for more context.

[jieter]

One last nitpick, and a test failure (slightly annoying false positive in the spell check, "middlewares" need to be added to https://github.com/zostera/django-modeltrans/blob/master/docs/spelling_wordlist.txt)

I'll merge and create a new release after these are resolved...

[jieter]

Thanks!