Add parser for Pocket API

Pass a url like `pocket://Username` to import that username's archived Pocket
library. Tokens need to be stored in ArchveBox.conf with the following keys:

```
POCKET_CONSUMER_KEY = key-from-custom-pocket-app
POCKET_ACCESS_TOKENS = {"YourUsername": "pocket-token-for-app"}
```

`POCKET_ACCESS_TOKENS` MUST be on a single line, or the JSON will be
misinterpreted by the parser as a new key/value pair.

setup.py

@@ -59,6 +59,7 @@ setuptools.setup(
         "python-crontab==2.5.1",
         "croniter==0.3.34",
         "w3lib==1.22.0",
+        "pocket==0.3.6",
         # Some/all of these will likely be added in the future:
         # wpull
         # pywb