add support for extensions in json1

[alx]

Hi Jean-Phillipe,

I need support for extensions on the json feed: https://jsonfeed.org/version/1#extensions

The new testcase is running fine.

Thanks for your work on this lib, and have a nice day :)

Alex

[coveralls]

Coverage increased (+0.2%) to 89.73% when pulling 29a7b90 on alx:json1_extensions into 3f46291 on jpmonette:master.

[coveralls]

Coverage increased (+0.2%) to 89.73% when pulling 777efdd on alx:json1_extensions into 3f46291 on jpmonette:master.

[coveralls]

Coverage increased (+0.4%) to 89.894% when pulling ea6d752 on alx:json1_extensions into 3f46291 on jpmonette:master.