Handle default language prefix

[st4lk]

Add following settings (all False by default):

SOLID_I18N_HANDLE_DEFAULT_PREFIX

If True, respond normally to url with default language prefix. Otherwise raise 404 (default behaviour).
Example:

LANGUAGE_CODE = 'en'
SOLID_I18N_HANDLE_DEFAULT_PREFIX = False # default
url '/' render default language
url '/en/' return 404

# ========
SOLID_I18N_HANDLE_DEFAULT_PREFIX = True
url '/' render default language
url '/en/' render default language (same as '/')

SOLID_I18N_DEFAULT_PREFIX_REDIRECT

If True, url with default language prefix will return redirect to url without any language prefix.
Example:

LANGUAGE_CODE = 'en'
SOLID_I18N_DEFAULT_PREFIX_REDIRECT = False # default
url '/' render default language
url '/en/' return 404

# ========
SOLID_I18N_HANDLE_DEFAULT_PREFIX = True
url '/' render default language
url '/en/' return redirect to '/'

Linked to #10